The first time Sherlock calls him friend, John corrects it. "Colleague," he says stiffly, because... "Well, I say friend." "He doesn't have friends." "You've met him. How many friends do you imagine he has?" He's not entirely sure what or who Sherlock is mocking, but mocking he must be, and John doesn't like being part of a mockery, thank you very much.
Sherlock seems to get the point, because the next time he introduces John he simply says, "My flatmate."
Then, after there's been shooting and running and solving and laughing and bickering and fighting and accepting and adjusting and living, there comes another introduction. "This is my friend-" Sherlock starts, and then he stops, glancing at John with rare uncertainty.
John simply nods and sticks out a hand to shake. "John Watson," he says.
A/N: Again, any kind of review would be nice; doesn't matter how long. ;)