Yes, HYSTERESIS is a Scrabble word!

Letter values in Scrabble US: H 4, Y 4, S 1, T 1, E 1, R 1, E 1, S 1, I 1, S 1.

Is it a valid word?

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

the lagging of an effect behind its cause; especially the phenomenon in which the magnetic induction of a ferromagnetic material lags behind the changing magnetic field