Describes something that exists, happens, or is used within an organization, system, or group, rather than externally. Often used in business or technical contexts.
This problem should be resolved internally.
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.