Is an ace worth 10?

Could you please clarify for me if an ace in a deck of cards, when used in a game like blackjack, is indeed valued as 10 points? I've heard differing opinions on this matter, and I'd like to ensure my understanding is correct. Is it a rule that is universally applied, or does it vary depending on the specific game or variant being played? Thank you for your time and assistance in clearing up this confusion.