A nomenclator was a person who would say the names of important people at meetings. What does this have to do with cryptography? Well, a nomenclator in cryptography is a type of substitution cipher.

Let us say that Bob wants to send a letter to Alice. She does not want Eve to be able to read it so she encrypts it with a substitution cipher. If Eve is trying to break the code, she can realize that Bob is probably using the word 'Alice' a lot when writing his letter.
Eve can find words that could be 'Alice' and could then know what the letters 'a', 'l', 'i', 'c', and 'e' are.

But then Bob could get smart and he and Alice could design a better substitution code. Let us say that this is their current key:

ABCDEFGHIJKLMNOPQRSTUVWXYZ
EDCRFGVBTHNYWSXQAZUJMPLOKI

To encrypt 'ALICE' they would write 'EYTCF'. But what if instead of writing out 'EYTCF' they just used the symbol '%' every time they wanted to say 'ALICE'. To encrypt 'BOB' they would write 'DXD' but what if they just used the symbol '*'.
This would make life complicated for Eve because now instead of their being 26 symbols she has to make sense of, there are 28. But why should 'ALICE' and 'BOB' be the only words with special symbols, why not replace the word 'THE' with '#' and replace 'AND' with '='. This idea of replacing some common words with symbols turned into the first nomenclator, a substitution cipher with some common words replaced as symbols.

Here is an example nomenclator I made.

ABCDEFGHIJKLMNOPQRSTUVWXYZ
EDCRFGVBTHNYWSXQAZUJMPLOKI

THE AND HE SHE IT YOU THEY ARE IS IF THEN
_%___&__+___=___>__?____!___~___/__}__\