The denotation of a word is its direct, literal or specific meaning (as can be found in a dictionary). If a word also has implied or associated meanings when used in a certain way, these are called the word's connotations. The word 'bat' in this sentence is being used with its denotation: 'A bat is a flying mammal.' however, the word, 'bat' can also take on extra meanings, often metaphorical, e.g. 'He went like a bat out of hell'.
Interestingly, the word 'bat' also happens to have several possible denotations: 'a cricket bat', 'a vampire bat', 'They bat next' (as well as other slang and dialect meanings): words that have several denotations are called polysemic. Polysemy is an area of semantics and pragmatics.