a close friend of the opposite sex that you spend a lot of time with and talk to often much the same as you would with a girl/boyfriend, though they never will become your girl/boyfriend due to factors such as, but not limited to: age difference, the other person being in a relationship, the other person living too far away, fear of ruining the friendship etc.
You will often to "date-like" things with them, such as: go to dinner and a movie, take them to a party to introduce them to your friends, or have them meet your parents. though you will never do other "date-like" things, such as: hold hands while walking, get/give a kiss goodnight, have them to a big family function like Christmas dinner etc.
can be a cause of grief and jealousy if one of the friends is in a relationship and their girl/boyfriend feels threatened by the surrogate friend.
NOT to be mistaken for a friend with benefits, there are no sexual conotations on a surrogate friend.
Bill: i know eh, they go to movies almost once a week, and Tim always pays. And they went on that camping trip on the long weekend. If someone didnt know better, they'd think Tim and Sharon were going out
Bob: Tim only wishes that was the case. Poor guy is hung up on her bad. Sharon is like his surrogate girlfriend
*having a surrogate girl/boyfriend sucks