Stemming from the word, Madeira, this word is a term for oxidization.
Madeira is a type of a fortified wine from Portugal and is produced by a combination of aging and heating. The wine is aged in heated tanks or in barrels and tucked away in attics in the subtropical climate. All the things you typically do not want to do to wines.
However, when a wine has been "maderized"and not maderized on purpose like Madeira, then these qualities are a fault. The fault is oxidation and often from the wine being in a hot environment, like storing your good French whites in the garage or on the outside patio during the summer.
Not only will the maderized wine take on a sherry-like oxidized taste, but you will also note a change in color of the wine, especially if it is a white wine and has taken on a dull brown color. Red wine will take on a dark copper color.
Sometimes maderisation may be desirable in the case of certain dessert wines, where the change occurs due to long bottle aging, such as that 1929 Château d’Yquem you have stored down in your basement.