Definition: Pre-cum (precum) or pre-ejaculatory fluid is a clear, mucus-like liquid that can be expelled through the urethra and out of the tip of the penis during male sexual arousal. It is secreted from the Cowper glands, and may contain both sperm and STDs. There is no way to control pre-cum, and the amount can vary from man to man and situation to situation.

http://sexuality.about.com/od/glossary/g/precum.htm