- In general English, a host is a person, organisation, animal, cell, computer or similar that receives guests or intruders.
- In biology, a host is an organism that harbors a parasite, mutual partner, or commensal partner, typically providing nourishment and shelter. See host (biology) for detail and also symbiosis.
- In psychology and mental health, a host is the most important (to therapeutic goals) mental entity in someone who has Dissociative Identity Disorder (DID, formerly known as Multiple Personality Disorder or MPD). See host (psychology) for detail;
- In Japan, a host is a male who provides an entertainment service for females. See host (japan) for detail.
- In informatics, host is synonymous with server.
may also be a group, such as an army or a large number of people under a single cause or banner. The typical usage in English is like "a host of US troops invaded Iraq in 2003
In the Bible, expressions like the heavenly host mean "chorus or army of angels". This is using a similar definition of host.
In Roman Catholicism, the host is the transubstantiated bread in the Eucharist. See also viaticum.