In dictionary entries, the letter n means

noun.
next.
new.
none.

noun.

Explanation:

In dictionary entries, there are abbreviations, in the case of n. (noun), that function as indicators of the different parts of speech of the English language (verb tense, grammar) the words defined belong.

Other abbreviations are used to indicate different subjects such as the origin of words, L. (latin); or the use of a word in specific circumstances, colloq. (colloquial).

Other examples of abbreviations are:

she is outspoken and direct