Unilateral is a legal term that is typically used to describe a type of contract. In a unilateral contract, one party offers to pay another party a fee for services rendered. However, the other party has no legal obligation to fulfill the contract. However, if that party does fulfill the contract, then the original party has a legal obligation to pay the other party.
Unilateral contracts are very different from other types of contracts. In many other types of contracts, both parties are obligated to perform duties, or pay money based on the terms of the contract. However, unilateral contracts are only paid if one party chooses to perform the duty. For example, a person offering a reward in a newspaper for a lost kitten is offering a form of a unilateral contract and will be obligated to pay if someone finds the kitten. However, no one is obligated to search for the kitten.