This is the 7017th most frequent Catalan word.
zer
"Zer" is not a Catalan word. It may be a typo or not exist in the Catalan language.
Here 'zer' is a proper noun referring to a specific letter in the Basque language.
El zer és una lletra de l'alfabet basc.
The zer is a letter of the Basque alphabet.
In this context, 'zer' is used to reference a term from another language, asked about its meaning.
Què és 'zer' en basc?
What is 'zer' in Basque?
This describes the meaning of 'zer' as a question word in the Basque language.