Reasonable

An innocuous-looking adjective that nonetheless exercises the wits of attorneys the world over, especially when modified by the word "commercially".

As long ago as 1947 Lord Greene MR issued the seminal words on the topic in the famous case of AP Picture Houses v Wednesbury: The court will not substitute its own judgment of reasonableness over that of the person charged with the obligation to act reasonably. The bar for overturning a decision is much higher: It must be proved to be unreasonable in the sense that the court considers it to be a decision that no reasonable body could have come to.

It also illustrates that even the Master of the Rolls can end a sentence with a preposition.

In the commercial context, one often encounters the formulation "acting in a commercially reasonable manner". There's a good article there, if you're interested.

See

• AP Picture Houses v Wednesbury (includes link to transcript)
• commercially reasonable manner
• Barclays v Unicredit