Theoretically; in theory — indicating something that holds true in principle, though practical constraints may prevent it.
理論上はこの方法で全員のデータを処理できるが、時間が足りない。
Theoretically, this method can process everyone's data, but there isn't enough time.
理論上は可能でも、実験では違う結果が出た。
Although it's possible in theory, experiments yielded different results.
その問題は理論上は解けるが、実際に解くのは難しい。
The problem is solvable in theory, but hard to solve in practice.
Used nominally to form phrases like “理論上の〜” meaning theoretical or pertaining to theory.
理論上の前提が間違っている可能性を検討した。
We examined the possibility that the theoretical assumptions were incorrect.
理論上の限界を踏まえて計画を立てる必要がある。
You need to plan taking into account theoretical limitations.