Yes, SECOND is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, C 3, O 1, N 1, D 2.

Is it a valid word?

SECOND is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

1/60 of a minute; the basic unit of time adopted under the Systeme International d'Unites
an indefinitely short time
the fielding position of the player on a baseball team who is stationed near the second of the bases in the infield
a particular point in time
following the first in an ordering or series
a 60th part of a minute of arc
the official attendant of a contestant in a duel or boxing match
a speech seconding a motion
the gear that has the second lowest forward gear ratio in the gear box of a motor vehicle
merchandise that has imperfections; usually sold at a reduced price without the brand name

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

Also: s, second base, second gear, 2nd, 2d.

Not playable in Scrabble US: ARCSECOND, SECONDMENT.