Yes, FOCI is a Scrabble word!

Letter values in Scrabble US: F 4, O 1, C 3, I 1.

Is it a valid word?

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

the concentration of attention or energy on something
maximum clarity or distinctness of an image rendered by an optical system
maximum clarity or distinctness of an idea
a central point or locus of an infection in an organism
special emphasis attached to something
a point of convergence of light (or other radiation) or a point from which it diverges
a fixed reference point on the concave side of a conic section

Also: focal point.