na-adjective
Describes something based on theory rather than practice or experience. Often used in academic or analytical contexts.
彼の説明は理論的でわかりにくい。
His explanation is theoretical and hard to understand.
理論的には可能だが、実際には難しい。
It's theoretically possible, but difficult in practice.
理論上 is an adverb meaning 'in theory', while 理論的 is an adjectival noun meaning 'theoretical'.
Compound of 理論 (theory) + 的 (-like, -ical).