indispensable; essential; vital
Used for something that is absolutely necessary and cannot be done without. Often appears in formal, business, or academic contexts.
His cooperation is indispensable for this project.
水は生命にとって不可欠な要素である。
Water is an essential element for life.
Accuracy is an indispensable skill for this job.
Water is essential for sustaining life.
A relationship of trust is an indispensable condition for successful negotiation.
不可欠 means 'indispensable' while 不可侵 means 'inviolable'; the shared 不可 prefix makes them easy to mix up despite very different meanings.