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.
Formed from 内部 (ないぶ, 'inside') + 的 (てき, '-like, -al'), a common pattern for creating adjectival nouns.