Transition friend is a person that brings together two or more people that would otherwise not actually hang out. Sometimes people get to know each other through the “transition friend”. 

“中间朋友”就是把两个（有时是两个以上）朋友叫到一起玩的人，如果没有他，这些人是不会结伴出游的。有时人们通过这种“中间朋友”得以结识彼此。

Example:

Adam: Jeremy, are you going to Jose's party tonight?

亚当：杰里米，你今晚要去约瑟的派对吗？

Jeremy: No, Alex is my transition friend for Jose. I can't just show up without him being there.

杰里米：不去，艾利克斯是我和约瑟的“中间朋友”。他不在那里的话我也不能去。

Adam: Oh, I got you.

亚当：哦，我明白你的意思了。