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Abbreviated Accounts: the accounts that some smaller and medium-sized companies may prepare and file for Companies House; if your company files abbreviated accounts at Companies House, you must still file full statutory accounts as part of your Company Tax Return.

Accounting Period: the period of time used to determine your company or organisation's taxable profit for Corporation Tax; normally matches your company or organisation's financial year.

Accounting Reference Date (ARD): the term used by Companies House to refer to the last day of your company's financial year; see annual report and accounts.

Active: an active company or organisation is liable for Corporation Tax; generally your company or organisation is active for Corporation Tax purposes when it is, for example, carrying on a business activity, trade or profession, buying and selling goods with a view to making a profit or surplus, providing services, earning interest, managing investments or receiving any other income.

Agent: an accountant, tax adviser or other professional that you appoint to deal with HMRC on your behalf for your tax affairs including Corporation Tax.

Amendment: you make an amendment to your Company Tax Return when you make a change to (amend) your return form or supporting documentation that you've already filed; HMRC can also make an amendment to your return.

Annual Investment Allowance (AIA): a type of capital allowance: for accounting periods that end after 31 March 2008; most companies and organisations can claim an AIA.

Annual Report and Accounts: also known as statutory accounts, financial accounts or audited accounts - the accounts that limited companies must provide to their members under the Companies Act and file at Companies House; you must also submit your company's annual report and accounts to HMRC as part of your Company Tax Return; if you choose to file abbreviated accounts at Companies House you must still submit the full statutory accounts to HMRC.

Apportion: if your Corporation Tax accounting period doesn't fall wholly within a single financial year (in other words it starts before and ends after 1 April) and different Corporation Tax rates applied in each, you'll need to divide or apportion your company or organisation's chargeable profits between the two financial years on a time basis (in days not months).

Assessment: if HMRC finds that your company or organisation hasn't paid enough Corporation Tax because, for example, you understated your taxable income or overstated your deductions or reliefs, they can make an assessment of the additional Corporation Tax you must pay.

Associated Company: a company is associated with another company if one is under the control of the other, or if both are under the control of the same person or persons; control is usually defined by reference to ownership of share capital, or voting power; a company may be an associated company no matter where it is resident for tax purposes.

Authorised Person: someone authorised by a company to sign the declaration on a Company Tax Return such as the company secretary, a director, or other officer of the company, an authorised agent, a liquidator or an administrator if one is acting.