"Entry Clearance" is a Visa or an Entry Certificate issued to a passenger before they travel to the UK. If you have Entry Clearance you will not be refused permission to enter the UK unless there has been a material change in circumstances, or false information was given, or you did not disclose important facts when the entry clearance was obtained. Holders of entry clearances may also be refused on medical grounds , if they have a criminal record , if they are subject to a deportation order ,or if there are any other exceptional reasons why they may not be admitted.

a type of visa issued at a foreign post that enables someone to enter the UK for a specific purpose and a specific length of time

a visa issued to an individual prior to travelling to the UK