The most fundamental law of a country or state. The term is capitalized only when referring to a specific constitution (e.g., U.S. Constitution, Texas Constitution, etc.).
Historically, a constitution did not need to be a single written document. Instead, the term might simply refer to the general structure of a certain government regardless of how it arose or evolved.
Increasingly, "constitution" refers to a single written document that explicitly creates government institutions, defines the scope of government power, and guarantees certain civil liberties. This is how the term gets used under U.S. law.