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From an immigration advisor in the UK:
It depends who you are talking to.
Entry clearance means the sticker in a passport granting someone permission to come to the UK for a specific purpose.
Technically a visa is entry clearance for a visa national. A visa national is someone travelling on one of the passports listed in Appendix 2 of Immigration Rules Appendix V: visitor rules. Entry clearance for a non-visa national is just called entry clearance.
However is common daily speech, βvisaβ is used to mean all kind of permission to be in the UK, including an actual visa as defined above, entry clearance in general, a residence permit showing leave to remain, or leave to enter granted on a stamp for a non-visa national visitor.