There's actually a linguistic term that describes this, and it's called zero derivation. We can get away with verbing (well, sorta) in English because we've lost most of our inflection: instead of determining word class from some modification of a root, we usually infer it from word order, particles, prepositions, and auxilliary verbs. This makes English an analytic language as opposed to a primarily synthetic language such as German or Russian.
In linguistics, derivational morphology is the study of how inflections change word class, e.g., adding -ish to create an adjective from a noun. (The study of how words change to reflect number, tense, possession, etc., but retain the same word class is called inflectional morphology). Thus, zero derivation means that a word has changed class without any morphological change.
As an aside, zero derivation also permits us to noun verbs, but no one gets bent out of shape by this.