Perfect season

A perfect season is a sports season, including any requisite playoff portion, in which a team remains and finishes undefeated. The feat is extremely rare at the professional level of any team sport but has occurred more commonly at the collegiate and scholastic levels in the United States. A perfect regular season is a season excluding any playoffs, where a team remains undefeated; it is less rare than a complete perfect season but still exceptional.
Exhibition games are generally not counted toward standings, for or against. For example, the 1972 Miami Dolphins lost three of preseason exhibition games but are considered to have had a perfect season.

Basketball

No National Basketball Association team has ever had a perfect season., the 2015–16 Golden State Warriors have the best ever regular-season record in the NBA, with a record of 73–9, breaking the 1995–96 Chicago Bulls record of 72–10. However, the Warriors lost the 2016 NBA Finals to the Cleveland Cavaliers.
Two NBA teams have had nearly perfect regular seasons at their home arenas:

The 1985–86 Boston Celtics finished with a 40–1 record at the Boston Garden. The Celtics' only home loss occurred on December 6, 1985 to the Portland Trail Blazers by the score of 121–103. The Celtics also won all 10 of their home games in the postseason to finish 50–1 at home.
The 2015–16 San Antonio Spurs also finished with a 40–1 at AT&T Center. The Spurs' only regular-season home loss occurred on April 10, 2016 to the Golden State Warriors by the score of 92–86. In the playoffs, however, the Spurs were 3-2 at home, losing twice to the Oklahoma City Thunder in the conference semifinals to finish 43-3 at home.

Basketball leagues outside the NBA tend to have fewer games per season, thus making a perfect season more achievable:

Khimik won the 2014–15 Ukrainian Basketball SuperLeague by winning all 30 regular season games, and winning all playoff games, for a 36–0 overall record.
The American Basketball Association has seen three perfect seasons:
* Shreveport-Bossier Mavericks
* Jacksonville Giants
In 2017, the three-on-three league BIG3, which featured an eight-game regular season and two-round playoff, had a perfect team in its inaugural season when Trilogy swept all ten games on their schedule.
Fenerbahçe Women's Basketball team scored a perfect season in the 2023–24 Women's Basketball Super League by winning all 28 season games. The team also won all the championships that they competed in during the 2023–24 season: Turkish Women's Basketball Cup, FIBA Europe SuperCup Women and EuroLeague Women.
Potawatomi Fire of The Basketball League had a perfect season in 2024
Raleigh Aces of the Women's American Basketball Association had a perfect season in 2024
Cricket

County cricket

English first-class county cricket has existed as the top tier of domestic cricket in England since the middle nineteenth century, and until the 1950s, it was up to the highest standard of the game. Seasons have varied in length: before the 1880s, they were generally less than ten matches in length and some "first-class" counties played only against one or two different opponents. Between 1887 and 1929, seasons were gradually increased in length to a standard twenty-eight matches for all counties. However, because of the development and popularity of one-day cricket, seasons have been reduced to twenty-four games in 1969 and twenty in 1972, though this was increased by two in 1977 and 1983. With an increase to four days for all games, sixteen or seventeen games have been played since 1993.
Also, because of improvements to pitches via the heavy roller and covering to protect from rain, the proportion of games "drawn" has steadily risen since the 1870s.
Since tables of results have been kept in 1864, the only team to have competed a true perfect season—winning outright every game—was Yorkshire in 1867 when led by George Freeman's and Tom Emmett's deadly fast bowling on uncovered and unrolled pitches, they won all seven county games.
Since 1868 numerous county teams in longer schedules have finished a season unbeaten, but none have managed to win every single game outright:

Season	Team	Wins	Losses	Draws	Remarks
1864	Surrey	6	0	2	Also defeated a combined England team by eight wickets
1876	Gloucestershire	5	0	3	W. G. Grace scored over 1,000 runs in August including the first two triple centuries in first-class cricket
1877	Gloucestershire	7	0	1	Defeated England by five wickets
1881	Lancashire	10	0	3	One match against Middlesex cancelled because Harrow Wanderers booked Lord's
1884	Nottinghamshire	9	0	1	Only draw against Surrey in August Bank Holiday game saw Surrey with three wickets in hand and 153 runs to win
1886	Nottinghamshire	7	0	7
1900	Yorkshire	16	0	12	Lost only two games, both at home to Somerset between 1900 and 1902. Did lose to MCC at Lord's with J. T. Hearne taking nine for 71 on a perfect pitch.
1904	Lancashire	16	0	10
1907	Nottinghamshire	15	0	4	One match abandoned against Yorkshire. Hallam and Wass took 298 wickets between them in very wet summer
1908	Yorkshire	16	0	12
1925	Yorkshire	21	0	11	Most games and most wins by unbeaten county team
1926	Yorkshire	14	0	17	Finished second, narrowly behind Lancashire who won seventeen games and lost two. Played seventy-one games without loss before losing to Warwickshire on May 23, 1927
1928	Lancashire	15	0	15	Went 35 games without loss and overall lost only once in eighty-three county games before losing to Sussex on May 24, 1929.
1928	Yorkshire	8	0	20	Finished fourth of seventeen teams Played fifty-six unbeaten county matches before losing to Kent on July 1, 1929, but only won fourteen of these
1930	Lancashire	10	0	18	Had lost only four of last 135 games at end of season.
1969	Glamorgan	11	0	13
1972	Warwickshire	9	0	11
1973	Hampshire	10	0	10
1974	Lancashire	5	0	15	Finished only eighth of seventeen teams Lowest win percentage by unbeaten county team
1998	Leicestershire	11	0	6	Lost only two games between 1996 and 1998
1999	Surrey	12	0	5	Last season of single-division Championship
2004	Warwickshire	5	0	11	First Division
2008	Warwickshire	5	0	11	Second Division
2009	Durham	8	0	8	First Division
2012	Yorkshire	5	0	11	Finished second in Second Division

American football

Pre-NFL era and competing leagues

Predecessors to the National Football League such as the Ohio League, New York Pro Football League and Western Pennsylvania Professional Football Circuit had many perfect seasons:

In western Pennsylvania, the 1900 and 1901 Homestead Library and Athletic Club teams and the 1903 Franklin Athletic Club had perfect seasons.
In Ohio, the Massillon Tigers, Akron Indians, Shelby Blues, and Dayton Triangles all had perfect seasons during this era.
In 1918, the Buffalo Niagaras went 5–0–0 in 1918, in a league that consisted of teams entirely from the city of Buffalo in 1918.
In 1920, the Union Club of Phoenixville played in a league consisting mostly of local teams and earned a perfect season, claiming for itself a mythical national championship.

The caliber of talent in these leagues was neither high nor consistent, the seasons were generally shorter than in the modern NFL, and it was common for top teams to play all their games at home while lesser teams played all of their games on the road. In 1918, for example, Dayton and Buffalo had the additional advantage of having its strongest competitors suspend operations due to the Spanish flu and the First World War, restrictions that also prevented the two teams from playing each other. Thus, it was much easier to earn a perfect season than it would become in the NFL.

1937 Los Angeles Bulldogs

In 1937, the Los Angeles Bulldogs joined the second American Football League after the Cleveland Rams defected to the NFL. Playing a combination of AFL teams and independent franchises, the team went 16–0, with 8 wins coming against AFL teams. The Pro Football Hall of Fame cites the Bulldogs’ dominance as one of the key factors in the AFL's demise, and as an independent team the next season, the Bulldogs finished with a 10–2–2 record, including a 2–1–2 record against NFL teams. Several of the team's players were invited to play on the "Pro All Stars" team in the NFL's first Pro All-Star Game in Los Angeles, and the Bulldogs are considered to be one of the few independent teams to have ever achieved parity with the NFL.