http://en.wikipedia.org/wiki/Pre-ejaculate
Pre-ejaculate (also known as pre-ejaculatory fluid or Cowper's fluid, and colloquially as precum) is the clear, colorless, viscous fluid that is issued from the urethra of a man's penis when he is sexually aroused. The fluid is usually secreted by Cowper's glands during arousal, masturbation, foreplay or at an early stage during sex, some time before the man fully reaches orgasm and semen is ejaculated.