A negated relation symbol indicating that one set is not a subset of another set in set theory.
Examples
Showing a set is not a subset
\{1, 2, 3\} \nsubseteq \{1, 2\}
Comparing number sets
\mathbb{R} \nsubseteq \mathbb{Q} \cup \{\pi\}
Vector space relationship
V_1 \nsubseteq W \text{ since } v \in V_1 \text{ but } v \notin W