What does "stability" mean in regards to sum and scalar multiplication?

Question

I need to provide a subset of $\mathbb R^2$ that is stable concerning scalar multiplication but not regarding summation, and the converse.

Question: What does "stability" in this context mean?

Answer 1

I guess it means here that if $x \in D \subset \mathbb{R}^2$ then for $\lambda \in \mathbb{R}^2$, ( and for $y \in D$ ) $$ \lambda x \in D \ \text{ and } \ x+y \notin D $$