• Australian Capital Territory, the capital territory of the Commonwealth of Australia
• ACT (examination), a college entrance exam in the United States

In business:

• Advanced Cell Technology, a biotech corporation involved in researching and developing cloning and stem cell technologies.
• Ada Core Technologies, a software company that develops an Ada compiler, tools and libraries
• Applied Computer Techniques, a computer services company, renamed Apricot Computers in 1985
• Aviation Composite Technology, an aircraft manufacturer in the Philippines
• ACT (Nasdaq), Automated Confirmation of Transactions, a trade reporting and clearing system
• Asia Container Terminals Ltd.

• Act of Parliament (Act of Congress in the USA) a statute or law passed by a legislature.
• Act (band), a British band
• Act (document), a document recording the legality of a transaction or contract
• Act (theater), a segment of a performance, such as a play or opera
• Acting, action of an actor in a theatrical performance
• ACT (examination), a college entrance examination
• ACT! is a customer relationship management (CRM) application
• Act (web browser), a type of web browser
• Australian Capital Territory, Australian seat of government
• Advanced Concepts Team, A Research Team of the European Space Agency
• Acceptance and Commitment Therapy, A psychological intervention
• Australian Cricket Team
• Act or S-act, the action of a monoid on a set, or a semiautomatonoun

CSTS or ACTS (Crew Space Transportation System and Advanced Crew Transportation System respectively) is a crew transportation system which is jointly studied by the European Space Agency (ESA), the Roskosmos Russian Space Agency (eventually in replacement of the Kliper project) and lately the Japanese Aerospace Exploration Agency (JAXA), with the objective to design a spacecraft for LEO operations such as servicing the International Space Station but also capable of exploration of the Moon and beyond. This study was conceived as a basic strategic plan to keep a viable European human space program alive because NASA officials have announced that NASA's Vision for Space Exploration Orion spacecraft will be developed without participation of international partners. According to press releases and ESA official Manuel Valls, JAXA has recently joined the European-Russian cooperationoun CSTS is currently in an initial study phase, which shall last for 18 months from September 2006 to spring 2008. The outcome of the study is planned to be presented to the ESA full member state conference in late 2008.