De Interpretatione Ch. 3 - "What is a Verb?" - Aristotle

Definition of a Verb
A verb is a sound which not only conveys a particular meaning but has a time-reference also. No part by itself has a meaning. It indicates always that something is said or asserted of something. Let me explain what I mean by 'it has a time-reference also.' Now, 'health' is a noun, for example, 'is healthy' is a verb, not a noun. For the latter conveys its own meaning but also conveys that the state signified (namely, health) now exists. Then, a verb was an indication of something asserted of something; I mean, of a something predicated of a subject or found present in it. 

Aristotle begins chapter three with the definition of a verb. He says that a verb is: "a sound which not only conveys a particular meaning but has a time-reference also. No part by itself has a meaning." In other words, a verb is (1) a sound (2) that has meaning through human agreement, not something inherent in the sound (3) is conjugated with reference to time (4) has no meaning in its parts other than by convention (5) makes a truth claim about the world. 

Verbs have time inherent in them because they assert some predicate of a subject, which is a truth claim about reality at a particular time. 

Indefinite Verbs

'Is not-ill,' 'is not-well' and so on I should not, for my own part, call verbs. Though they certainly have the time-reference and function at all times as predicates, I know of no recognized name. Let us call them (for want of a better) by the name of indefinite verbs, since we use them of all kinds of things, non-existent as well as existent. 

Just like indefinite nouns and names, indefinite verbs exist but are not quite the same thing as definite verbs because we only know what they don't refer to, not what they actually refer to. 

Tenses of Verbs
'He was healthy' or 'he will be healthy' I likewise should not call a verb. I should call it the tense of a verb. Verb and tenses in this respect differ: the verb indicates present time but the tenses all times save the present. 

The changing of the same verb to indicate different times does not mean it's a different verb altogether, this is simply the differing tense of the same verb in question. 

Verbs Need a Subject to Make Truth Claims

Verbs by themselves, then, are nouns, and they stand for or signify something, for the speaker stops his process of thinking and the mind of the hearer acquiesces. However, they do not as yet express positive or negative judgements. For even the infinitives 'to be,' 'not to be,' and the participle 'being' are indicative only of fact, if and when something further is added. They indicate nothing themselves but imply a copulation or synthesis, which we can hardly conceive of apart from the things thus combined."

Verbs are some expression of the verb to be which makes a positive or negative claim about reality only when subjects are included with it. Verbs by themselves simply are nouns.