Refers to the central, essential, or most important part of something, similar to English 'core'. Used in both concrete and abstract contexts.
この問題のコアは何ですか。
What is the core of this problem?
Colloquial adjectival use describing something as extreme, intense, or deeply committed, often in the context of hobbies, fandoms, or subcultures.
彼はコアなゲーマーだ。
He is a hardcore gamer.
This new technology will likely become the core of the company.
地球のコアは非常に高温だ。
The Earth's core is extremely hot.
このバンドはコアなファンに支えられている。
This band is supported by passionate fans.
彼はそのバンドのコアなファンだ。
He's a hardcore fan of that band.
Borrowed from English 'core'. The adjectival sense developed in Japanese colloquial usage, influenced by English expressions like 'hardcore'.