A named class that things are sorted into. Its Greek root is TO SPEAK AGAINST - to categorise was first to accuse, and only later to assert.
Greek kategoria (an accusation, a statement against someone), from kategorein (kata-, against + agoreuein, to speak publicly, from agora, the assembly place); Aristotle repurposed the word for his fundamental classes of being, and 'to speak against' softened into 'to assert' along the way.
A named class that things are sorted into — the word's own history is a reminder that naming a class was once an act of accusation, not a neutral filing.