A voluntary agreement between two competent parties is known as:

A valid contract is defined as a voluntary agreement between two competent parties that meets all necessary legal requirements and is enforceable in a court of law. To be considered valid, a contract must have mutual assent (offer and acceptance), a lawful object, and consideration (something of value exchanged).

In the context of the choices provided, a valid contract signifies that both parties involved have willingly agreed to the terms, are capable of entering into the agreement, and that the contract complies with relevant legal standards. This type of contract can be upheld and executed within the legal framework, providing protection and obligations for both parties.

The other options describe different contract conditions that do not support the notion of a valid agreement: an unenforceable contract may exist but lacks the ability to be enforced in court due to some defect, a void contract is not valid from the moment it is created, and an invalid contract is one that fails to meet the essential elements necessary for a contract at any point. Thus, it's clear that a valid contract is the only choice that accurately reflects the definition of a voluntary agreement between two competent parties.