Definition
Posse has two distinct legal meanings that developed along separate tracks and are now largely independent of each other.
1. POSSIBILITY (Latin: posse, "to be able"). In classical legal Latin, posse signifies possibility or potential existence, as opposed to actual existence. The phrase in posse describes something that may come into being; in esse describes something that currently exists. A child not yet conceived may be an heir in posse; once born, that child holds the status in esse. This usage appears primarily in older equity and property reasoning, particularly in discussions of future interests, contingent remainders, and unborn beneficiaries.
2. ENFORCEMENT FORCE (short for posse comitatus). In its more practically durable sense, posse refers to a body of persons summoned by a sheriff or other peace officer to assist in the execution of legal process, the suppression of civil disturbance, or the pursuit of a fugitive. The full phrase is posse comitatus — Latin for "the power of the county" — but in common legal usage posse alone refers to this summoned force. At common law, the sheriff's authority to call out the posse was broad: any able-bodied adult male could be conscripted into service. The Posse Comitatus Act of 1878 (18 U.S.C. § 1385) later restricted the use of federal military personnel in this role, codifying a firm boundary between civilian law enforcement and the military.
Common Language
Modern common usage (Wiktionary): A group summoned to help law enforcement; a search party; more loosely, a criminal gang or peer group.
Historical common usage (Webster's 1913): "See Posse comitatus" — Webster's treated the legal phrase as primary and the standalone term as derivative.
The gap is instructive in reverse here: ordinary modern English has absorbed the enforcement sense of posse (westerns, popular culture) while almost entirely losing the philosophical-legal sense of possibility. A researcher encountering posse in an early equity opinion may be reading about contingent interests and future estates, not sheriffs and fugitives. Context is essential.
Common Confusion
The two meanings — possibility and enforcement force — are etymologically connected (both derive from Latin posse, "to be able") but function as wholly independent legal concepts. Conflating them produces serious misreading of historical texts. Additionally, researchers sometimes treat posse and posse comitatus as interchangeable in the enforcement context, which is generally accurate in older sources but can cause confusion when a source uses posse alone in its philosophical sense. The Posse Comitatus Act of 1878 applies specifically to federal military forces and does not use the abbreviated form; researchers should not assume that every modern reference to the Act's restrictions flows through the common-law posse doctrine.
Recognized Forms
/SUBTYPES
In posse: In possibility; not yet in actual existence. Regularly contrasted with in esse (in actual being) and occasionally with in fieri (in the process of coming into existence). Found heavily in treatises on future interests, wills, and equity.
Posse comitatus: The power of the county; specifically, the body of persons a sheriff may summon to assist in executing process or maintaining order. The source of the Posse Comitatus Act's name and the enforcement doctrine's formal designation.
Why It Matters in Research
Researchers working in the Law Mind corpus will encounter posse in two very different bibliographic neighborhoods, and context determines everything.
In early chancery materials, equity treatises, and property law texts — particularly those addressing contingent remainders, executory interests, or unborn heirs — posse almost certainly signals the philosophical in posse / in esse distinction. Searching this material for enforcement doctrine will produce false hits.
In criminal law, local government, and constitutional law materials, posse signals the sheriff's summoning authority. Here the critical research trap is anachronism: the Posse Comitatus Act of 1878 dramatically narrowed what had been a broad common-law power, but pre-1878 sources describe a much more expansive doctrine. A treatise from 1840 on the sheriff's posse authority describes a legal world that federal statute substantially altered. Researchers should timestamp their sources carefully.
The Posse Comitatus Act also generates its own distinct research trail in military law, national security law, and emergency powers scholarship. The encyclopedia entry linked above is the appropriate starting point for that thread. Do not conflate the common-law doctrine with the federal statutory restriction — they share a name but occupy different bodies of law.
For corpus searching: posse alone is ambiguous; posse comitatus is more reliable for enforcement-doctrine materials; in posse is the reliable signal for the possibility sense.
Historical Dictionary Support
The historical dictionaries agree on both meanings but weight them differently. Burrill's is the most complete, offering both the philosophical sense (with reference to Cowell and Wharton's Lex) and the enforcement sense, and is the only source to note the phrase pro posse suo ("to the extent of his power"), a usage that appears in Bracton and in older pleading contexts. Bouvier's gives the clearest functional account of the sheriff's authority and connects the enforcement doctrine to the writ context. Anderson's is notable for cross-referencing CONTINGENCY, which correctly places the in posse / in esse distinction within its proper analytical framework in equity and property law.
Black's (both editions) treats posse in its philosophical sense only, which reflects the dictionary's emphasis on doctrinal legal Latin rather than procedural practice — a gap researchers should note. Rapalje & Lawrence's entry is oddly prefaced with a cross-reference to "portable" (a pagination artifact from the print source) and adds nothing beyond the standard formulation.
None of the historical dictionaries adequately addresses the 1878 statutory development or its effect on the common-law doctrine. For any research question touching federal military authority or post-Reconstruction enforcement powers, the historical dictionaries must be supplemented with sources addressing the Act directly.
Jurisdictional Note
The common-law posse comitatus doctrine remains part of state law in most American jurisdictions, and sheriffs in many states retain statutory authority to summon assistance. The Posse Comitatus Act restricts federal military forces; it does not displace state law on civilian enforcement posses. Researchers should not assume that federal restrictions translate into equivalent state limitations without jurisdiction-specific verification.
Encyclopedia Cross-Reference
Posse Comitatus Act — Restrictions on Domestic Military Operations and Exceptions (The Law Mind Military, Veterans & Admiralty Law Encyclopedia)