half life vs half-life

half life and half-life both are nouns.

Word	Noun	Adjective	Verb	Adverb
half life	Yes	No	No	No
half-life	Yes	No	No	No

As nouns, half life and half-life are synonyms defined as:

half life and half-life: the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)

half life (noun)	half-life (noun)
the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)	the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)