vertical bar on vector

Question

I am reading a paper and I have problem to understand the equation (this is the full paper)

Assume that a one-dimensional discrete-time signal s of length N exhibits sparsity in certain orthonormal basis Ψ defined by the basis vectors $\Psi = [\Psi_{1}|\Psi_{2}|\Psi_{3}|...\Psi_{N}]$. Therefore, the signal s can be represented using its sparse transform domain vector x as follows:

What is the meaning of the vertical bar in $\Psi = [\Psi_{1}|\Psi_{2}|\Psi_{3}|...\Psi_{N}]$?

Answer 1

The context is:

Assume that a one-dimensional discrete-time signal $s$ of length $N$ exhibits sparsity in certain orthonormal basis $\Psi$ defined by the basis vectors $\Psi=[\Psi_1∣\Psi_2∣\Psi_3∣\ldots\Psi_N]$.

So, it seems that $\Psi$ is an orthonormal basis, that the vectors of that basis are $\Psi_1,\Psi_2,\Psi_3,\ldots,\Psi_N$ and that $[\Psi_1∣\Psi_2∣\Psi_3∣\ldots\Psi_N]$ means $\{\Psi_1,\Psi_2,\Psi_3,\ldots,\Psi_N\}$.