What is SEM?

Wikipedia has a very informative article about search engine marketing.

Geez. I was all ready to talk about scanning electron microscopy.
We use the SEM to determine the source of contaminants (metal filings, dirt, brass) in oils and fuels.

Did you mean Search Engine Marketing?

It is placing key words into your Web site or Web pages to riase their level on search engines.

It also encompasses paid placements.

Structural Equation Modeling?

SEM is a TLA with multiple DIM requiring RTFM in re LRF’s.

Search Engine Marketing is a form of Search Engine Optimization, one in which spammers and cretins insert unrelated keywords into their sites in order to elevate their sites in search engines’ rankings, thereby breaking the search engines for ordinary users.