The term compound is used to describe the combining of similar grammatical units in a number of circumstances.
A compound sentence is a sentence made up of two or more independent clauses. The clauses are joined by conjunctions and/or distinctive punctuation marks.
A compound subject is a subject made up of two or more different subjects acting on the same predicate.
A compound verb is a simple predicate with two or more different verbs showing different actions or conditions.
Both compound subjects and compound verbs are joined by conjunctions, sometimes with additional punctuation.
A compound modifier is two or more adjectives or adverbs modifying a single word or phrase. They are normally joined by conjunctions or punctuation marks.
A compound word is a single word made up of two or more distinct words combined into that single word. The word blackbird is a compound word made up of the words black and bird.