A "Solo Ad" is an ad which is sent out to subscribers to an ezine, but without any other content. In other words, it's like a special issue of an ezine which simply contains a single ad.
Here is example of http://www.imleaks.net/solo-ads-to-10k-and-50k-im-list-becuase-the-money-is-in-the-list-guaranteed-clicks/ service.