Which term describes obligations become excessively costly or difficult?

The term that refers to obligations becoming excessively costly or difficult is commercial impracticability. This legal concept arises when unforeseen events render a contractual obligation much more burdensome than what was originally contemplated by the parties involved. In such cases, a party may be excused from performing their contractual duties if they can demonstrate that the circumstances that arose were truly beyond their control and not something they could have reasonably predicted when the contract was made.

Commercial impracticability does not imply a complete impossibility of fulfilling the contract, but rather that the performance is substantially more difficult or expensive than expected. Therefore, it provides a means for parties to seek relief from their obligations under a contract when the conditions have drastically changed.

Other terms that might seem related, like impossibility of performance, refer to situations where it's completely unfeasible to perform a contract due to unforeseen circumstances. Waiver involves the voluntary relinquishment of a known right, while release pertains to the formal relinquishment of an obligation under a contract. These don't accurately address the nuance of increased costs or difficulties associated with fulfilling a contract.