The grammatical construction of a sentence in which the subject does not do the acting, but rather the action is performed on it (e.g., the boy was bitten by the dog). While active sentences are preferred in that they make for clearer and more interesting writing (a fairly frequent comment from editors of journals!), there are occasions when passive ones are to be preferred. One example is when the person performing an action is unknown (the message was sent to me yesterday), and another when the ‘doer’ is not important (e.g., the company ran out of supplies). In the teaching of grammar, children are typically encouraged to use the active voice when writing.
See Action syntax, Syntax