mature: Porn niche involving performer(s) over the age of 40. A popular emerging sub-niche of the mature niche are MILFs or Sexy Moms.
hd: A BDSM abbreviation for humiliation and discipline. Also short for High Definition
big ass: For big booty and massive buttocks lovers.
ass: Video genre that focus on the butts. It can includes anal, asslicking and rimming and even spanking, but also more generic movies that just focuses on asses, big or small.