Nicknames for the Scoobies:

Season 1 (Giles, Willow, Buffy & Xander):

the Slayerettes (Willow, TW), the bad element (Snyder, TPS)

Season 2 (Giles, Willow, Buffy, Xander, Angel, Jenny and eventually Cordelia):

kids (Jenny, WSWB), The Three Musketeers (Cordelia, WSWB), little undead playgroup (Cordelia, SAR), the archeology club (Willow, lying to Ampata about what the Scoobies are doing, IMG), the Scooby Gang (Xander, WML1), tweakos (Cordelia, WML1), Buffy referred to Scoobies as Dr. Laura for the deceased

Season 3

not company-plate people (Buffy, Dead Man's Party)

Name: The Scoobies

They started out as the Slayerettes in Season One, from Willow's quote in TW:

Buffy: You guys don't have to get involved.
Xander: What d'ya mean? We're a team! Aren't we a team?
Willow: Yeah! You're the Slayer, and we're, like, the Slayerettes!

Over the years, the people have changed, but the definition is the same: Any group of people who help the Buffy Summers, the Slayer, save the world are members of the Scooby Gang or the Slayerettes.

Ever Changing Scoobies:

In the beginning, they were Willow, Xander and Giles.