Object
An object is a nonliving, distinct thing. Composite things, like buildings, comprise more than one object. See also “Breaking Objects.”
Occupied Space
A space is occupied if a creature is in it or if it is completely filled by objects.
Opportunity Attacks
You can make an Opportunity Attack when a creature that you can see leaves your reach using its action, its Bonus Action, its Reaction, or one of its speeds. To make the Opportunity Attack, take a Reaction to make one melee attack with a weapon or an Unarmed Strike against the provoking creature.