Both these words have been used interchangeably. A lot of sites list marinade as a variant word for marinate. We plan to set the record straight. Though connected, but both the words have a completely different meaning and here is the explanation:
Marinade is a noun and is a liquid mixture (usually vinegar, yogurt, oil, herbs & spices) in which meat or chicken is soaked before cooking.
Marinate on the other hand is a verb and refers to the process of soaking the meat or chicken in the Marinade.
Thus, you Marinate the meat or chicken in a Marinade.