Entry clearance is the process whereby a British High Commission or Embassy in another country grants you permission to enter the UK. Commonly referred to as a visa, entry clearance provides evidence [..]

This is permission to enter the UK.  It is the name of the visa that you are given at the start of any period of time in the UK.  It is usually a yellow sticker in your passport. See picture.

This is the name for a visa issued overseas to enable a foreign national to travel to the UK. It is in the form of a sticker placed in a passport by a British Embassy or High Commission.

Permission to enter the UK obtained in advance of coming to the UK. Entry clearance takes two forms; "visas" (for visa nationals) and "entry certificates" (for non-visa nationals).