Tilde is what we say in English for the squiggley mark over the letter ñ. We get our word from the Spanish word.
La tilde, or
un acento ortográfico, is used to mark certain vowels. There's a good write-up
here about how to use them.
Words that need to distinguished, because they are homonyms, carry a tilde, too, but this is called
un acento diacrítica.
A couple of sources say the squiggley mark over the ñ is also called
la tilde, while others say it is called
la virgulilla. Let's see what Alfonso calls it.