Let $M$ be a smooth manifold with or without boundary. A vector field on $M$ is simply a smooth section of the projection $\pi : TM \to M$. The space of all vector fields on $M$ may be denoted by (see Wikipedia) $$\Gamma(TM) \qquad \mathrm{Vect}(M) \qquad \mathfrak{X}(M)$$ Whereas the first two notations are quite intuitive (one can argue about the letter $\Gamma$ for sections), the last one seems a bit off for me. I know, that sometimes notations are just get established because they are handy, like in this case short, nonetheless I quite wonder, where this notation originates from. So to state my question:
What is the meaning behind the notation $\mathfrak{X}(M)$, i.e. fraktur $X$, for the space of smooth vector fields on a manifold $M$?