Geist is actually a reservoir in the northeast corner of the Indianapolis area. "The Geist area" simply refers to the area around it, including northeast Lawrence Township in Indianapolis, and Fall Creek Township of Hamilton County in the eastern part of the Fishers area. 96th Street, which separates Indianapolis from Hamilton County to the north, runs through the middle of the reservoir. Hence, many of the residential parts of the Geist area are in Lawrence Township, while many are actually in the Fall Creek Township portion of Fishers. Since "Geist" is not an official governmental unit, I do not have population statistics for the area. However, it is approximately the size of Carmel in population, and is a very popular area.
The Lawrence Township part of the area (south of 96th Street) is served by Lawrence Township Schools, while the Fall Creek Township part of the area (north of 96th Street) is served by Hamilton Southeastern Schools.