marinade: (noun) an acidic liquid, typically made of vinegar or wine with spices, used for soaking food to flavour or tenderize it
marinate: (verb) to soak something in a marinade
The increased use of marinade as a verb in the last decade has led some dictionaries to accept it as an alternative, but it is still unacceptable to many. My recommendation, for the sake of clarity and to avoid harsh judgement from language purists, is to use marinate as the verb. Marinate has been in use as a verb since the 1640s.