Learn English – the abbreviation for ‘who are’

The n itself does not stand for a sound. The entire abbreviation “Defⁿ” stands for the sound of the word definition.

Could it be you are thinking of “Defⁿ” as a contraction? A contraction is a word in its own right. For example, isn’t is a word, originally coined by contraction of the words is not.

Abbreviations are not contractions. They are still read as the original word. For example, you might abbreviate singular as “sing.” but you would still pronounce it singular.

There really are no general rules for how to abbreviate or how to write an abbreviation, but some words are almost always abbreviated the same way and it is convenient for the reader if you adopt the common abbreviation. For example it is common in academic works to abbreviate pages as “pp.” But that is not a matter of English spelling or grammar, just written style.

The letter "X" has long been used as a place-holder.

It was once standard for illiterate persons to sign legal documents with an "X".

Mathematicians use the letter "x" quite often as a place-holder.

For example, a mathematician might write, "Ɐ x, y, z ∈ ℝ, (x = y) ⋏ (y =z) ⟹ (x = z)"

It is standard to use an "x" to mean, "replace the x with some other string of symbols".

It is a convention.