Wikis are subject to SemanticSpam when a particular vendor comes by and tries to hawk their wares. However, owing to the public nature of most wikis, this may not be spam, but simply communication from the corporation to the community. For instance, on MeatballWiki, we have had a vendor describe their OpenSpaceTechnology services. Since MeatballWiki was interested in the subject, it is on-topic for a vendor to politely introduce themselves.
SemanticSpam on a wiki would most obviously be blatantly OffTopic material, such as Viagra ads. However, it may also include on-topic material that is simply presented in a very rude way. Vendors: IntroduceYourself? first.
By contrast, in the post-Google Internet, there is now LinkSpam.
This is a very simple description. -- SunirShah