What do you call a group of words that function as a single part of speech

A group of words that does not contain a subject and predicate and acts as one unit as a part of speech (noun phrase, verb phrase, prepositional phrase, verbal phrase).

Explanation:

the unsaturated fatty acid is a fat consist of one or more double bond in the fatty acid chain. if it contains one double bond then called as monounsaturated fatty acid however if, it contains more than one bonds called as a polyunsaturated fatty acid. true fatty acids are called unsaturated fatty acids as it reduces the risk of high blood cholesterol and good for health.