A brood is a large emergence of Magicicada that takes place in the same area, in the same year, at predictable intervals (note that broods aren't a biological fact but rather a useful human invention). Broods are divided into 17-year cycles and 13-year cycles.
Broods are cross-species. If I say "all 17-year species", then that will be Magicicada cassini, Magicicada septendecim, and Magicicada septendecula. If I say "all 13-year species", then it will be Magicicada neotredecim, Magicicada tredecim, Magicicada tredecassini, and Magicicada tredecula.
Our numbering system comes to us from Charles Marlatt, who enumerated broods via Roman numerals. Broods I-XVII are reserved for 17-year broods and Broods XVIII-XXX are for 13-year broods. Thus, there is 30 hypothetical broods; only 15 are extant, with the rest either being extinct or "spurious" (meaning that they are not recognized by the scientific community and are false observations made by Marlatt).
Brood I is centered around the Shenandoah Valley, with minor populations in Blue Ridge Parkway and Massanutten Mountain and disconnected populations in Long Island and the Bristol, VA/TN area. All 17-year broods are in this brood, and the next emergence is in 2029.
Brood II is spread across the eastern seaboard of the United States, with disconnected populations in northeastern Georgia and Oklahoma (for whatever reason). All 17-year species emerge, EXCEPT in Connecticut, where M. septendecim is the only cicada (EXCEPT for around North Branford, which also has M. septendecula). The next emergence is in 2030.
Brood III spans Iowa, Missouri, and northwestern Illinois. All 17-year broods emerge, with the next emergence in 2031.
Brood IV spans Kansas, Oklahoma, Missouri, Iowa, and Nebraska. All 17-year broods emerge, with M. septendecim and M. septendcula decreasing in the westward areas of the brood. The next emergence is in 2032.
Interesting fact: M. cassini in this brood have an orange striping under their abdomen, which isn't a shared trait that other M. cassini have!
Brood V spans a wide, diaganal line from Lake Erie to the Viriginian Appalachias, with a disconnected population in Long Island and Jim Thorpe, PA. All 17-year species emerge, and the next emergence is in 2033.
There's no common name for this brood; maybe the "West Virginian Brood" given the fact that almost the entire state of West Virginia has the brood?
Brood VI is centered around the Brushy Mountains with a pretty debated range; we really only know that the Brushy Mountains section is correct. M. septendecim and M. septendecula emerge, with the next emergence being in 2034.
Brood VII is centered around the Onondaga Nation with a now-extinct population near Rochester, NY. Only M. septendecim is in this brood, and the next emergence is in 2035.
Brood VIII is located in eastern Ohio, the West Virginian panhandle, and western Pennsylvania. All 17-year species emerge, with the next emergence being in 2036.
Brood VIII also doesn't have a name, so maybe the "Western Pennsylvanian Brood"? The University of Connecticut describes it as being centered around the panhandle of West Virginia which I guess is true but it's 100% mainly Pennsylvanian.
Brood IX is located in upland North Carolina, Virginia, and West Virginia, primarily around the northern half of the Blue Ridge Mountains. All 17-year species emerge, with the next emergence in 2037.
Maybe Brood IX could be called the "Blue Ridge Brood"?
Brood X has multiple sections: 1) Illinois, Indiana, Ohio, Kentucky, and Michigan, 2) The Northeast Corridor region, and 3) North Carolina and Georgia. A disconnected population is located on Long Island. All 17-year species emerge, except in Michigan which generally only has M. septendecim and has a small M. cassini population. The next emergence is in 2038.
Brood XI is a now extinct brood located in eastern Connecticut and a doubtful population in Rhode Island. It consisted only of M. septendecim, with its last emergence being 1954. It most likely went extinct as a result of climate change and being at the natural end of the Magicicada range.
Brood XII is a spurious brood. While we are here, the other spurious broods are Brood XV (most likely stragglers from Brood II), Brood XVI, XVII, XVIII, XX, XXIV, XXVII, XXIX, and XXX.
Brood XIII spans Illinois, Iowa, Wisconsin, and Lake County, IN. All 17-year species emerge, with the next emergence being in 2041.
Brood XIV has a large span centered around Kentucky and Tennessee, with a second large population in central Pennsylvania, and two small disconnected populations in Long Island and Cape Cod. All 17-year species emerge, with the next emergence being in 2025.
Brood XIX is our first 13-year brood! It is the largest brood by geographic extent, making a sort of "U" shape, beginning in Iowa and Illinois, spreading down to the Deep South states, and swinging back up to Washington D.C.. All 13-year species emerge (all four in the same place is only in the contact zone, which is around Missouri and Illinois). The next emergence is in 2037.
Brood XXI is a now extinct brood from the Apalachicola River Valley. 1870 was the last recorded emergence; what cicadas belonged to the brood is unknown.
Brood XXII is centered around Baton Rouge, LA with disconnected populations in southern Ohio and northern Kentucky. M. tredecim, M. tredecassini, and M. tredecula emerge, with the next emergence being in 2027.
Brood XXIII is centered around the Mississippi River, with a disconnected population in Weldon Springs State Park, IL. All 13-year species emerge, and the next emergence is in 2028.