A constitution is the most fundamental law of a sovereign body. The term is capitalized only when referring to a specific constitution (e.g., U.S. Constitution, Texas Constitution, etc.).
In many cases, "constitution" refers to a single written document that explicitly creates government institutions, defines the scope of government power, and guarantees certain civil liberties.
Nonetheless, a constitution does not inherently need to be written. In the case of an unwritten constitution, the term might simply refer to the general structure of a certain government regardless of how it arose or evolved. For example, the United Kingdom does not have any written constitution.
[Last updated in August of 2022 by the Wex Definitions Team]