I read sites about Dota 2, and there they often mention 'units'. I'm confused because 'units' here doesn't mean 'distance units', but can mean many things. So, could you explain what the term refers to in Dota 2?
A unit is any entity that can be selected and is not a building or a ward. Creeps are units, heroes are units, summons are units, illusions are units.
Note that this definition is not specific to Dota 2, and is used by the vast majority of RTS games.