First class generally indicates a high level of service or importance. Specific uses of the term include:

• First class travel
• First class mail
• First-class cricket
• First Class Scout (Boy Scouts of America)
• First Class Diver – the highest grade of diver in the original (and now obsolete) classification system of the British Sub Aqua Club.
• First class honours degree – see British undergraduate degree classification

In mathematics: First class is a particularly high quality travel class offered by passenger airlines, railways and shipping companies. ... First-class cricket matches are those of at least three days length in which both teams have two innings each, and which involve either international teams or the highest division of domestic competition. ... First Class Scout is a rank attainable by a Scout in the Boy Scouts of America (BSA) and ranks above Second class and below Star. ... The British Sub-Aqua Club or BSAC is the governing body of recreational diving in Britain. ... The British undergraduate degree classification system is a grading scheme used to distinguish between the achievements of undergraduate degree holders (such as those gaining bachelors degrees or undergraduate masters degrees) in the United Kingdom. ...

• First class constraint

In computing: In Hamiltonian mechanics, lets say we have a symplectic manifold M with a smooth Hamiltonian over it (for field theories, M would be infinite-dimensional). ...

• First class (computing)
• First-class object
• First class function
• FirstClass - E-mail and Online Conferencing System