An accusative verb is a verb that would normally take a direct object, but which can also be used intransitively, without change of meaning. Some examples in English are eat, see, win. For example:
In languages such as Latin or German which have grammatical cases, the word apple in the first example would be in the accusative case (that is to say, it would have the form which is used for the object of a verb). So an 'accusative verb' is one which would have an object in the accusative case if used transitively.
Accusative verbs can be distinguished from unaccusative verbs. These are intransitive verbs which indicate a change of state (usually involuntary) like the following:
The first two verbs above can be used with an object, but with a change of meaning (e.g. 'broke something' vs. 'got broken itself').
Accusative verbs can also be distinguished from unergative verbs, which are intransitive verbs which don't describe a change of state, but which also do not usually take a direct object, e.g.: