luck vs toss-up

luck and toss-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
luck	Yes	No	No	No
toss-up	Yes	No	No	No

As nouns, toss-up is a hyponym of luck; that is, toss-up is a word with a more specific, narrower meaning than luck:

luck: an unknown and unpredictable phenomenon that causes an event to result one way rather than another
toss-up: an unpredictable phenomenon

Other hyponyms of luck include bad luck, mischance, mishap, even chance, tossup.

luck (noun)	toss-up (noun)
an unknown and unpredictable phenomenon that causes an event to result one way rather than another	an unpredictable phenomenon
an unknown and unpredictable phenomenon that leads to a favorable outcome
your overall circumstances or condition in life (including everything that happens to you)