Tilde
 

I have always known the tilde to be the squiggle over the N forming the Ñ.
Now, I have noted that the tilde is a word for the accent mark. Is that a correct usage. What's the word for the mark over the N?

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.

I think tilde is mostly used for acento ortográfico. When learning to place tildes is very common to hear: todas las palabras tienen acento, pero sólo algunas llevan tilde. So, you use these words to contrast two meanings: acento fónico / tónico and acento ortográfico.
I heard of virgulilla with the nuance Rusty says, but I don't think it's widely used since you don't need to call anything an inseparable part of a letter.
Signo diacrítico (literally, two criteria / two meanings) is also used, but I think it's a more technical concept, as it implies a difference in the meaning of the word. So, there are some tildes that are also signos diacríticos and some that are not.

As you all already know, the ñ is a letter in its own right, and not an n with a line over it, thus it has its own name and the squiggle over it doesn't have much need to have a name. As already mentioned, the acute accent mark used over some vowels in Spanish is often referred to as a tilde.

tilde
 

I have some English-speaking friends who argue with me that tilde is the little line over the Ñ. I guess they were taught that BUT that is not true. Oh boy, they would get mad and argue with me...LOL...

La letra Ñ es una sola y no hay necesidad de nombrar la linea que lleva encima.

Tilde is definitively what you write over a vowel to show acento ortrográfico.

According to DRAE, tilde is any mark you write over a letter to distinguish it from another letter, including the "ñ" mark and, especially, the marks you use for acentuación.
So tilde is both. But it's completely unnecessary to refer to the mark over the "ñ" as tilde. On the other hand, it's really common to call tilde to the acento ortográfico, as many people have been taught in Spain.
I know scholarship tradition varies from country to country but, according to what your friends say, how do they distinguish acento from tilde?: Todas las palabras tienen acento, pero sólo algunas llevan tilde.

Es que, como sabes, ñ no existe en inglés. Cuando me enseñaron español
la marca sobre el n era tilde (imagino por el propósito de enseñar nada más). Es claro que esa marca no necesita una palabra aparte en español porque ñ es una letra distincta.

Gracias por explicarmelo.

Yo creo que está bien decir que tilde como una palabra en inglés se refiere a la línea sobre la ñ, porque así se usa en el inglés americano.

Agreed. We who are exposed to Spanish names, street names, place names, etc. that contain the letter ñ always have to distinguish the letter by saying something like an n with a tilde (over it) or an n with a squiggley (curvy, wavy) mark. Tilde is an English word (pronounced TILL-duh) we borrowed from Spanish. The symbol itself has many other uses in science (especially mathematics and computer sciences). It started appearing on keyboards ~1960 (circa 1960). It was added to the ASCII standard in 1965.

I looked it up on<snip> and this is what they came up with 1. Word:- tilde
Pronunciation:- [teel’-day]
Meaning(s):- f.

1: Tilde (sobre la ñ).
2: Accent (acento).
3: Dot or dash over a letter.
3: Iota, a tittle (cosa insignificante).