Is the notation $\text{ch}$ standard for some function? What does it mean?

Question

A paper I'm reading has the following formula:

$\ln ( 2 (\text{ch}(t') - cos ( \theta))] = t' - \sum_{k \in \mathbb{Z}^*} \frac{ e^{-|k|t'}}{|k|}e^{ik \theta}$.

Which they call the Fourier representation. They also 'set'

$2 \text{ch}(t_n) = 4 + m^2 - 2 cos(\theta_n)$. (I think this might be their definition of $t_n$ though.)

Does anyone know what $\text{ch}$ is? This is the first time this notation appears in the paper. I guess it must be some standard function?

However, I couldn't find it here: https://en.wikipedia.org/wiki/List_of_mathematical_functions

The paper is "Multiple Hamiltonian Walks on Manhattan Lattice" by Duplantier and David.

Answer 1

Okay, I found it. [I'd say delete this question, but it might be helpful to keep up.]

It's an alternative notation for hyperbolic cosine. See: https://en.wikipedia.org/wiki/Hyperbolic_function

"The abbreviations sh, ch, th, cth are also at disposition, their use depending more on personal preference of mathematics of influence than on the local language. "