What does the \ notation mean in geometric structures plane separation postulate?

Question

Referring to what the Plane Separation Postulate says about half-planes $H_1$ and $H_2$, it notes $H_1\cup H_2=\mathbb{P}\backslash l$. What does the \ mean between $\mathbb{P}$ and $l$?

Plane Separation Postulate: For every line $l$, the points that do not lie on $l$ form two disjoint, nonempty sets $H_1$ and $H_2$, called half-planes bounded by $l$, such that the following conditions are satisfied:

1. Each of $H_1$ and $H_2$ is convex.
2. If $P\in H_1$ and $Q\in H_2$, then $\overline{PQ}$ intersects $l$.

Here, in symbols, is what the postulate says about $H_1$ and $H_2$:

1. $H_1\cup H_2=\mathbb{P}\backslash l$.
2. etc.