long tail

An SEO strategy whereby lots of low-volume search keywords in a market are targetted. In highly competitive markets with well-established top tier players, it can be difficult to get attention by going after the most popular search terms when several players with larger budgets can dominate simply by producing high quality content in volumes that smaller companies don't have the budget to sustain.
Smaller players could try to produce viral content, which is a bit of a crap shoot.

Alternatively, they could target lots of search phrases that the established players are ignoring.

This might lead to capturing business directly.

Oddly, a few prominent SEO practitioners and companies seem to have very different ideas about what a "long tail" SEO strategy is. However, the descriptions offered were so complex and muddled that I wasn't able to divine a reasonable second definition. I suspect that those discussions may have been the output product of content spinners, and that there really isn't a strong contender for these alternate approaches to a long tail SEO strategy.