sneak preview vs preview

sneak preview and preview both are nouns.

Word	Noun	Adjective	Verb	Adverb
sneak preview	Yes	No	No	No
preview	Yes	No	Yes	No

As nouns, preview is a hypernym of sneak preview; that is, preview is a word with a broader meaning than sneak preview:

sneak preview: a preview to test audience reactions
preview: a screening for a select audience in advance of release for the general public

sneak preview (noun)	preview (noun)
a preview to test audience reactions	a screening for a select audience in advance of release for the general public
	an advertisement consisting of short scenes from a motion picture that will appear in the near future

sneak preview (verb)	preview (verb)
	watch (a movie or play) before it is released to the general public