In web hosting, unlimited usually comes up when referring to storage space and bandwidth. There is no such thing as unlimited data transfer or diskspace as a web server has physical hardware limitations. Unlimited is a marketing term. Web hosts should really use the term unmetered instead.
Unlimited could also refer to other web hosting features including databases, email accounts, domains, etc. This is not technically possible either as both hardware and software restrictions would prevent a hosting plan truly having any unlimited features.