Mis·sis·sip·pi·an 
(mĭs′ĭ-sĭpē-ən)
(mĭs′ĭ-sĭpē-ən)adj.
1. Of or relating to the state or residents of Mississippi or the Mississippi River.
2. Of, relating to, or being the period of geologic time from about 359 to 318 million years ago, the fifth period of the Paleozoic Era. The Mississippian Period is characterized by the submergence of extensive land areas under shallow seas and by the appearance of primitive conifers. See Table at geologic time.
3. Of or relating to a Native American culture flourishing throughout the southern Mississippi Valley and much of southeastern North America from about 1000 AD until contact with Europeans, characterized especially by the establishment of permanent towns, settled agriculture, regional chiefdoms, and the construction of large earthworks serving as ceremonial centers.
n.
1. A native or resident of Mississippi.
2. The Mississippian Period. Also called Lower Carboniferous.
[Adj., sense 2, and n., sense 2, from the fact that rocks dating from the period are exposed in the Mississippi Valley.]