relaxation vs lie-in

relaxation and lie-in both are nouns.

Word	Noun	Adjective	Verb	Adverb
relaxation	Yes	No	No	No
lie-in	Yes	No	No	No

As nouns, lie-in is a hyponym of relaxation; that is, lie-in is a word with a more specific, narrower meaning than relaxation:

Other hyponyms of relaxation include bed rest, bedrest, laziness, dormancy, quiescence, quiescency, sleeping, leisure.

relaxation (noun)	lie-in (noun)
freedom from activity (work or strain or responsibility)	a long stay in bed in the morning
the act of making less strict
a method of solving simultaneous equations by guessing a solution and then reducing the errors that result by successive approximations until all the errors are less than some specified amount
an occurrence of control or strength weakening
a feeling of refreshing tranquility and an absence of tension or worry
(physics) the exponential return of a system to equilibrium after a disturbance
(physiology) the gradual lengthening of inactive muscle or muscle fibers