terrain flight vs low level flight

terrain flight and low level flight both are nouns.

Word	Noun	Adjective	Verb	Adverb
terrain flight	Yes	No	No	No
low level flight	Yes	No	No	No

As nouns, terrain flight and low level flight are synonyms defined as:

terrain flight (noun)	low level flight (noun)
flight at very low altitudes	flight at very low altitudes