A sports league is a structured collective of individual athletes, teams, or clubs, formally organized for competitive play within a specific sport to accrue points and ultimately determine an overall champion. These entities span a spectrum from localized amateur groups to complex, multi-million dollar international professional organizations. Technically, a "league" specifically implies an organization where participants are "in league" together; thus, competitions owned and controlled by external bodies rather than their participant members are often considered misnomers. Alternative terms such as "association," "conference," "division," or "series" frequently describe such organizations, even encompassing competitions structured like tournaments, exemplified by the UEFA Champions League which combines round-robin stages with a single-elimination format.

Leagues and broader league systems primarily adhere to two organizational paradigms. The first, prevalent globally, utilizes a promotion and relegation system, creating a hierarchical pyramid where top-performing teams ascend to higher divisions and lower-ranked teams descend, fostering an interconnected competitive environment. In contrast, the second model, predominantly found in Australia, Canada, and the United States, operates as a closed system. This structure features a fixed roster of teams with occasional expansion or relocation, critically lacking the movement between major and minor leagues seen in promotion/relegation models. Within these closed systems, internal subdivisions like conferences (e.g., NFL's National Football Conference and American Football Conference) and further geographical divisions are common, influencing scheduling and competitive dynamics.

A particularly unique and historically rooted organizational model is Major League Baseball (MLB), which serves as an umbrella entity for two distinct "major leagues": the American League (AL) and the National League (NL). These are designated as separate leagues rather than mere conferences due to their independent historical trajectories, differing financial influence, largely separate competitive schedules until recent interleague play, and their former status as distinct legal entities until 2000. This unique structure impacts conventional nomenclature, often requiring phrases like "in all of baseball" to denote the overarching entity, and necessitates parallel awards (e.g., separate MVPs and Cy Young Award winners for each league), underscoring a foundational structural separation despite operating under a unified MLB banner. Ultimately, all league structures share the core objective of establishing a non-random, scheduled competition ("season") leading to the systematic determination of a champion, often through round-robin play or subsequent playoff formats.