SEM (Search Engine Marketing)
Act of marketing a website via search engines, whether this be improving rank in organic listings, purchasing paid listings or a combination of these and other search engine-related activities. (Source: SEMPO)

SEO (Search Engine Optimization)
Act of altering a website so that it does well in the organic, crawler-based listings of search engines. (Source: SEMPO)

Site stickiness
Measure used to gauge the ability of a web site to hold a visitor's attention.

Skyscraper
Creative with dimensions 120x600 or 160x600 pixels.

Spam (a.k.a. unsolicited commercial e-mail, bulk e-mail, mass e-mail, junk e-mail)
Electronic message advertising goods or services that is sent to someone without his prior consent and in the absence of a previous relationship (including newsgroup, IRC channel, or message board postings unrelated to the topic of discussion of the particular newsgroup, channel or message board).

Superstitial
Interstitial format developed by Unicast which is fully pre-cached before playing; specs are 550 x 480 pixels (2/3 of screen), up to 100K file size and up to 20 seconds in length. (Source: IAB)