any method of trying to manipulate an environment using "flooding" techniques.  In terms of SEO, Spam can take the form of things such as:

• Keyword Stuffing of meta tags
• Keyword Stuffing of Keyword Density in on-page copy
• Link Spam
• Email Spam

Email spam for example is the practice of sending massive amounts of E-mail advertisements to people that have not asked for it. Many times, SPAM E-mail lists are created by harvesting E-mail addresses from various places on the Internet. SPAM is particularly notorious for its' association with low quality untargeted 'mass marketing' and for scams.