A marid is a type of malevolent spirit found in Arabic mythology and folklore. The word in Arabic, مريض, literally means "illness," while the Semitic root comes from the Hebrew marad, מרד, which means, among other things, "to rebel."
As can be guessed by the name, they are considered to be malevolent spirits, or demons, or type of jinn on par with the ifrit. They are mentioned in the Quran as "rebellious devils" who are trying to eavesdrop on Heaven, but to a western audience, they are likely better known for their role in A Thousand and One Arabian Nights where, depending on the translation, translator, and edition, they crop up as the go-to term for general-application evil spirits.
Or, if you're like me, you know them as a type of djinn in Jonathan Stroud's Bartimaeus Trilogy.