The essential factor that differentiates a bilateral contract from a unilateral contract is:

A bilateral contract is characterized by the exchange of promises between two parties, where each party commits to fulfill their obligations. This mutuality of obligation is what distinguishes it from a unilateral contract, in which only one party makes a promise that the other party can accept by performing a specific act.

In a bilateral contract, both parties have a vested interest in the agreement, and they are both legally bound to perform their respective promises. For instance, if one party promises to sell a car and the other party promises to pay a certain amount for it, both parties are entering into a bilateral agreement where they are each responsible for fulfilling their end of the bargain.

The other aspects mentioned, such as the number of parties involved, the length of the agreement, and the legal jurisdiction, do not fundamentally define the nature of the contract itself. The core element that sets apart bilateral contracts from unilateral ones is indeed the exchange of promises, solidifying the framework in which both parties operate within the agreement.