1. To act as a substitute for someone or something. ("Sub" is an abbreviation of "substitute.") I'm subbing for Eric for the next few days while he recovers in the hospital. Don't believe anyone who tries to convince you that low-fat cooking sprays can sub for real butter or olive oil and not taste any different.

2. To use, employ, or instruct someone or something to act as a substitute for someone or something else. In this usage, a noun or pronoun is used between "sub" and "for." With a healthy 30-point lead, the coach subbed the rookie for the star quarter back to give him some experience on the field. Let's try subbing an AC motor for the DC one we've been using and see if that improves the results.