280 (number)

280 is the natural number after 279 and before 281.

In mathematics

The denominator of the eighth harmonic number, 280 is an octagonal number. 280 is the smallest octagonal number that is a half of another octagonal number.
There are 280 plane trees with ten nodes.
As a consequence of this, 18 people around a round table can shake hands with each other in non-crossing ways, in 280 different ways.