Pseudoscience is a pretended or spurious science; a collection of related beliefs about the world mistakenly regarded as being based on scientific method or as having the status that scientific truths now have. — The Oxford English Dictionary (2nd edition)
This category comprises highly notable topics that are generally considered pseudoscientific by the scientific community (such as Astrology) and topics that, while perhaps notable, have very few followers and are obviously pseudoscientific (such as the modern belief in a Flat Earth).
The term pseudoscience itself is contested by a number of different groups for a number of different reasons.