The word is Spanish for ripe. In practice it describes the darkest usable range of wrapper shades, between colorado maduro and the near-black oscuro, though no standards body defines the boundaries and two makers may disagree about the same leaf.
How the colour happens
Extended fermentation breaks down starches and raises sugar concentration at the leaf surface. The heat is managed in the pilón — the stacked bulk of leaf — and a maduro cycle simply runs longer and hotter than one destined for a lighter shade. Thicker, more durable leaves are chosen because thin ones do not survive the process.
What it does and does not mean
Maduro is often assumed to mean strong. It does not: strength comes overwhelmingly from the filler, and a maduro wrapper contributes sweetness and body more than nicotine. It is one of the most persistent misreadings in the category.
The plume question
White crystalline bloom on a maduro wrapper is frequently reported as plume. Most of it is mould. See the separate entry for the distinction, which is visual and reversible in one case and not in the other.