Definition: A tax on the taxable profits of limited companies, organisations and other unincorporated entities in the United Kingdom.
The taxable profits of a limited company or organization are subject to the corporation tax.
Profits that are considered to be 'taxable profits' include any profit made on a company's taxable income and capital gains (aka chargeable gains).
UK based companies and organizations have to pay a corporation tax on their taxable profits, no matter where the profits came from (e.g. even if from overseas).
If a company has opertions in the UK but is not based there (because it's main office or branch is located outside of the UK) then the corporation tax is only charged on the taxable profits that arise inside the UK.
You can learn more about corporation tax on the HM Revenue and Customs website.