The Associated Press does God's work and defines "mom jeans" in a story about first lady Michelle Obama:
Mom jeans are an `80s style that is widely considered unflattering to women. They have a light wash, a high waist, fit loosely and are tapered at the ankle.
The AP's writeup, based on an interview with Obama on NBC's "Today," includes a quote from the first lady saying she's "not a mom jeans kind of mom."