Yes, LAMBDA is a Scrabble word!

Letter values in Scrabble US: L 1, A 1, M 3, B 3, D 2, A 1.

Is it a valid word?

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

the 11th letter of the Greek alphabet
the craniometric point at the junction of the sagittal and lamboid sutures of the skull