How do you call a word that is so rudimentary that it can't be defined without being redundant?
I think axiom should fit, but according to its official definition, an axiom is a statement that is taken to be true, and as far as I know, a word can't make an statement by its own.
Axiomatic is pretty good. Irreducible is another. However, I would argue there are no undescribable words. If that were true, no one could learn certain words in other languages
There are many, MANY words that do not have a 1 to 1 relationship between languages. Love is actually one of the more basic concepts most all languages have a word for.
The areas languages gets really interesting are in the idioms. THOSE are the expressions that generally take many words to explain.
Which is why if you want to write easily translatable text, never use idioms.
As far as I understand, we were not speaking about struggles with translating from one language to another, but about explaining a word within the original language.
Then I defer to an earlier answer from someone: ALL words have meanings and definitions. The only way one fails to describe a word is when they do not understand the word.
As Einstein said: "If you cannot explain it to a six year old, you do not understand it yourself."
He was speaking about relativity. If a simple word stumps you, I have bad news...