The lowercase Greek letter, lambda, carries several meanings. First of all, it represents scales, and thus balance.

The Greeks considered balance to be the constant adjustment
necessary to keep opposing forces from overcoming each other. The hook at the bottom of the right leg of the lambda represents the action required to reach and maintain balance. To the Spartans, the lambda meant unity. They felt that society should never infringe on anyone's individuality and freedom. The Romans adopted the letter to represent "the light of knowledge shed into the darkness of ignorance." Finally, in physics the symbol designates an energy exchange. Thus the lambda, with all its meanings, is an especially apt symbol for the gay liberation movement, which energetically seeks a balance in society and which strives through enlightenment to secure equal rights for homosexual people.

-Joseph P. Goodwin, "More Man Than You'll Ever Be: Gay Folklore and Acculturation in Middle America"  (Indianapolis: Indiana University Press, 1989) p. 26