A "prop" is any item, either hand held or free standing, that is not nailed down on a theatrical or film set.
The term originates from the
basic inventory of the company being the "property" of the owner.
General use is to give authenticity to a performance by adding a prop to give the actor "
business". Interacting with a brush, sitting at a dressing
table in a scene sets up the "suspension of disbelief" along with sets and costuming.