trial vs trial balloon

trial and trial balloon both are nouns.

Word	Noun	Adjective	Verb	Adverb
trial	Yes	No	No	No
trial balloon	Yes	No	No	No

As nouns, trial balloon is a hyponym of trial; that is, trial balloon is a word with a more specific, narrower meaning than trial:

Other hyponyms of trial include field test, field trial, alpha test, beta test, road test.

trial (noun)	trial balloon (noun)
the act of testing something	a balloon sent up to test air currents
(law) the determination of a person's innocence or guilt by due process of law	a test of public opinion
the act of undergoing testing
trying something to find out about it
an annoying or frustrating or catastrophic event
(sports) a preliminary competition to determine qualifications