luck vs tossup

luck and tossup both are nouns.

Word	Noun	Adjective	Verb	Adverb
luck	Yes	No	No	No
tossup	Yes	No	No	No

As nouns, tossup is a hyponym of luck; that is, tossup 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
tossup: an unpredictable phenomenon

Other hyponyms of luck include bad luck, mischance, mishap, even chance, toss-up.

luck (noun)	tossup (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)