second vs heartbeat

second and heartbeat both are nouns.

Word	Noun	Adjective	Verb	Adverb
second	Yes	Yes	Yes	Yes
heartbeat	Yes	No	No	No

As nouns, heartbeat is a hyponym of second; that is, heartbeat is a word with a more specific, narrower meaning than second:

second: an indefinitely short time
heartbeat: a very short time (as the time it takes the eye to blink or the heart to beat)

Other hyponyms of second include blink of an eye, flash, instant, jiffy, New York minute, split second, trice, twinkling, wink.

second (noun)	heartbeat (noun)
the fielding position of the player on a baseball team who is stationed near the second of the bases in the infield	an animating or vital unifying force
the gear that has the second lowest forward gear ratio in the gear box of a motor vehicle	the rhythmic contraction and expansion of the arteries with each beat of the heart
merchandise that has imperfections; usually sold at a reduced price without the brand name	a very short time (as the time it takes the eye to blink or the heart to beat)
a speech seconding a motion
the official attendant of a contestant in a duel or boxing match
a 60th part of a minute of arc
following the first in an ordering or series
1/60 of a minute; the basic unit of time adopted under the Systeme International d'Unites
an indefinitely short time
a particular point in time

second (adjective)	heartbeat (adjective)
a part or voice or instrument or orchestra section lower in pitch than or subordinate to the first
coming next after the first in position in space or time or degree or magnitude

second (verb)	heartbeat (verb)
give support or one's approval to
transfer an employee to a different, temporary assignment

second (adverb)	heartbeat (adverb)
in the second place