Sentence:  In the sentence ‘The mouse is eaten by the cat in the kitchen.', ‘The mouse' is the <WSD>subject</WSD>, ‘the cat' being the agent. 
Meaning of the word marked with <WSD> tags:  In a clause: the word or word group (usually a noun phrase) that is dealt with. In active clauses with verbs denoting an action, the '''subject''' and the actor are usually the same.