Styāna

Styāna or thīna is a Buddhist term that is translated as "sloth", "lethargy", "gloominess", etc. In the Mahayana tradition, styāna is defined as a mental factor that causes the mind to be withdrawn, unclear, and unable to focus. Thīna is defined as sluggishness or dullness of mind, characterized by a lack of driving power. In the Theravada tradition, thīna is said to occur in conjunction with middha, which is defined as a morbid state that is characterized by unwieldiness, lack of energy, and opposition to wholesome activity. The two mental factors in conjunction are expressed as thīna-middha.
Styāna or thīna is identified as:

One of the five hindrances to meditation practice
One of the Mental [factors (Buddhism)#Fourteen unwholesome mental factors|fourteen unwholesome mental factors] within the Theravada Abhidharma teachings
Closely related to the Sanskrit term kausīdya, that is identified as one of the twenty secondary unwholesome factors within the Mahayana Abhidharma teachings

Definitions

Theravada

Bhikkhu Bodhi explains:
The Atthasālinī states about sloth and torpor: “Absence of striving, difficulty through inability, is the meaning.” We then read the following definitions of sloth and torpor:
Nina van Gorkom explains:

Mahayana

The Abhidharma-samuccaya states:
Mipham Rinpoche states:
Alexander Berzin explains: