na-adjective
Describes something that exists, happens, or is used within an organization, system, or group, rather than externally. Often used in business or technical contexts.
この問題は内部的に解決すべきだ。
This problem should be resolved internally.
内部的には賛成だが、公には言えない。
Internally I agree, but I can't say so publicly.
外部 is the antonym meaning 'outside' or 'external', while 外部的 (がいぶてき) is the corresponding adjectival noun.
Formed from 内部 (ないぶ, 'inside') + 的 (てき, '-like, -al'), a common pattern for creating adjectival nouns.