Several specialized usages of the terms compact and compactness exist.

• Multiple definitions of the term "compact" are found in mathematics:
  • The most common usage relates to topology, where one considers compact spaces. That article also includes the closely related notion of a compact set.
  • The compactness theorem is a key theorem in the formal logic of propositional and predicate calculus.
  • In order theory and domain theory, the different (though not entirely unrelated) notion of a compact element is typically used.
• Compact flash is a type of data storage device.
• Compact car is an automobile size class in the American market.
• Compact disc is an optical disc used to store digital data, originally developed for storing digital audio.
• A British newspaper term, see compact (newspaper).