A "Wiki Word" is a text fragment that starts with a capital letter, one or more lowercase letters (or - or _), at least one more uppercase letter, and doesn't contain spaces.
WikiWords is a feature activated in about all TikiWikis. TikiWiki's forums, Wiki, blogs, etc. associate a hyperlink with words that match the "Wiki Word" syntax. The hyperlink points to an internal Wiki page of the same name.
If you want to link to a page which name doesn't use the WikiWord syntax, use the alternative syntax ((Page Name Here)) in order to generate hyperlinks to internal Wiki pages. This will work in most cases (depending the site's configuration).
WikiWords are a distinctive element of Wikis. The web's first Wiki used only "wiki words" for Wiki page names. More modern Wikis allow just about any sort of naming convention.
HotWords are a similar concept, using the WikiWord syntax but without necessarily linking to a Wiki page. They must be defined by administrators. As you look at the Last changes module of this site (tikiwiki.org) you realize that almost all pages are named so that they can be linked to using WikiWords. This way, it was possible to link to the page HotWords at the beginning of this paragraph without any extra "effort". WikiWords can even generate without writers being aware of it. TikiWiki itself is a WikiWord and that's why it was named this way
WikiWords are activated in the WikiSettings page.