(n) a friend with whom one has a mutual understanding that sometimes there's an inate need to cuddle and be physically close to someone, and in the absence of a relationship, he/she can depend on the other person to provide that physical closeness in a safe, friendly environment with no reasonable expectation of complications, drama or progression of the friendship to "something more"
A: I can't believer you stayed the night at Mark's! Are you dating?
B: Pfff. No. He's just my cuddle buddy.