noun
topic
TAH-pik/ˈtɑːpɪk/
A subject discussed, written about, or referred to.
The model bridge is the topic of their discussion.

Etymology
Greek topos meant a place, but also a subject of speech. Things pertaining to such 'places' became common headings for finding arguments, and then subjects discussed or written about.
In context
it's a complex topic anytime we're talking about anything related to memory it it's really complex