By definition an idiom is an expression or phrase that cannot be understood by the sum of its parts. There are, of course, many idioms that we can understand by virtue of their transparency like "shed light on" or "in a nut shell". But there are a number of categories that are really rather cryptic.
Question: Can we define phrasal verbs as idioms, or should we keep them separate in their own linguistic and lexical species?
There's some disagreement on just what does and doesn't count as a phrasal verb. All though are not compositional - we can't determine the meaning purely by breaking apart the verb and particle (or verb and preposition if you allow those to be considered phrasal verbs). Attempting to do so will either not give us a meaningful interpretation, or at best give us one with a wider definition than how the phrasal verb is actually used.
As such, it seems pretty clearly to be a type of idiom.
(Sometimes idiom is taken to mean any common figure of speech, even if it is taken literally and can be understood compositionally. While such a meaning would generally be considered incorrect, we can note that phrasal verbs match this too).
