olemus
(a) topoloogilise ruumi aluskomponent

Wiktionary:
3. (topology) most generally, a member of the topology of a given topological space

http://www.solitaryroad.com/c779.html

https://math.stackexchange.com/questions/2152735/what-does-open-set-mean-in-the-concept-of-a-topology/2152742

(b) meetrilise ruumi alamhulk,
mille iga punkt on selle ruumi sisepunkt
= a subset of a metric space, every point of which is an interior point of that space

http://people.math.sc.edu/sharpley/math555/Lectures/MetricSpaceTopol.html

https://en.wikipedia.org/wiki/Open_set