Definition
In law, bilateral describes a legal relationship, instrument, or obligation that runs in both directions — both parties assume binding duties to the other. The term appears most frequently in contract law but extends to records and international agreements.
1. Bilateral contract. A contract in which each party makes an enforceable promise to the other. The exchange of promises is itself the consideration: Party A promises to perform in exchange for Party B's promise to perform. Both sides carry executory obligations from the moment of formation.
2. Bilateral record. A record in evidence law that operates between parties and their privies — those who stand in a recognized legal relationship to an original party. Distinguished from a unilateral record, which may be offered only to establish a prima facie fact as against or for a stranger to the proceeding.
3. Bilateral agreement (international and administrative contexts). An agreement between exactly two sovereigns, entities, or parties, as distinguished from a multilateral agreement involving three or more. Usage in this sense tracks the ordinary English meaning closely.
Common Language
Modern common usage (Wiktionary): Having two sides; involving both sides equally; binding on both of the two parties involved.
Historical common usage (Webster's 1913): Having two sides; arranged upon two sides; affecting two sides or two parties.
The common and legal meanings are close but not identical in the critical respect. Ordinary usage treats "bilateral" as broadly descriptive — something that involves two parties or two sides. The legal meaning is narrower and more precise: in contract law, bilateral does not simply mean "two parties are present" but that both parties bear executory promissory obligations. A contract can involve two parties and still be unilateral if only one party makes a promise and the other's obligation arises by operation of law or through performance rather than promise.
Common Confusion
Bilateral vs. unilateral contract. The distinction is not about how many parties signed the document or are bound at completion — it is about how the contract is formed and what each party is promising at the moment of formation. In a unilateral contract, one party makes a promise; the other party accepts by performing an act rather than by making a counter-promise. In a bilateral contract, acceptance is itself a promise. Historically, courts struggled to classify option contracts and offers for reward under this framework; modern contract theory (reflected in the Restatement Second) softens the rigid bilateral/unilateral divide, treating many ambiguous offers as inviting either mode of acceptance. Researchers reading older cases should not assume the classification maps cleanly onto modern doctrine.
Why It Matters in Research
The bilateral/unilateral contract distinction is foundational to formation analysis in contract law and generates significant case law around offer, acceptance, and the moment at which obligations attach. When reading older American cases — roughly pre-1930 — the distinction was applied more rigidly, and courts sometimes denied enforcement of partially performed unilateral offers on grounds that no contract had yet formed. Modern authorities treat that outcome as unjust and often reclassify or use estoppel to avoid it. A researcher relying on pre-Restatement Second authority should note this doctrinal shift carefully.
The bilateral record / unilateral record distinction in evidence is largely a historical artifact of common-law evidence doctrine. It appears in older treatises and cases dealing with the admissibility of judgments and public records. Researchers encountering this usage in nineteenth- or early twentieth-century sources should not conflate it with the contract-law sense.
In international law and administrative practice, bilateral agreement is used descriptively without doctrinal weight — it means two parties, full stop. Research in those bodies of law requires no adjustment for this term.
Historical Dictionary Support
Anderson's definition efficiently captures the core distinction: a bilateral contract is "executory on both sides" — both parties have yet-to-be-performed obligations — while a unilateral contract leaves one party's obligation to legal implication rather than express agreement. Anderson's example of a guaranty as unilateral reflects the traditional view that a guarantor's obligation flows from the law's treatment of suretyship rather than from a mutually exchanged promise. This framing is consistent with classical common-law contract theory.
Anderson also records the evidentiary sense — bilateral and unilateral records — which most modern dictionaries omit. Researchers working in historical evidence sources will find this usage alive in treatises on judgment records and estoppel by record.
What Anderson's entry does not address is the doctrinal erosion of the bilateral/unilateral distinction in modern contract law. The entry reflects a period when the classification functioned as a hard rule. Contemporary sources treat it more as a descriptive tool than a determinative test.
Encyclopedia Cross-Reference
Contracts — Classification (Bilateral, Unilateral, Express, Implied, Quasi-Contract), The Law Mind Contracts & Commercial Law Encyclopedia