Part of speech
A word class(//) or part of speech is a categorisation of words into classes.
There may be some overlap between classes, and some grammar schemata differ on the inclusion or exclusion of classes. Modern, scientific grammar tends to try to minimise the number of parts of speech that words may be assigned to maximise parsimony. For example, "yesterday" is recognised as a pronoun in modern grammar; traditional analysis labels it an adjective and noun.