Existing; already in existence. Often used in the pattern 'existing ~' to refer to preexisting or conventional things.
既存の建物を改修してオフィスにする予定だ。
We plan to renovate an existing building to turn it into offices.
新しい機能は既存のシステムと互換性があるべきだ。
The new feature should be compatible with the existing system.
既存の枠組みを見直す必要がある。
It is necessary to review the existing framework.