what does the latin letter 'updown e' mean used in LSTM formular?

Question

I come cross with formular contains such symbol a lot when I was reading articls about machine learning, it is a 'updown e' does anyone know what does it mean?

this one I got is from a artical about LSTM

I assume it is someting like delta? I even dont know how to pronouse it, forgive me this might be a very stupid question.

Here is one equation like:

Here is the full artical which explains it:

Answer 1

Oh, it is the symbol for a partial derivative if you have a function of with multiple variables, e.g. E=f(?1,?2,...). Check https://en.wikipedia.org/wiki/Partial_derivative#:~:text=In%20mathematics%2C%20a%20partial%20derivative,vector%20calculus%20and%20differential%20geometry.