Level Set
A level sets is defined as
$$\mathcal{LS} := \{ x \in \mathbb{R}^n \, | \, f(x) \leq 0 \}$$
$$\mathcal{LS} := \{ x \in \mathbb{R}^n \, | \, f(x) < 0 \}$$
$$\mathcal{LS} := \{ x \in \mathbb{R}^n \, | \, f(x) = 0 \}$$
More information in Section 2.2.2.3 in the CORA manual.