Base Line - An east-west line parallel to the equator that is established at the initial point (origin) of a meridian. See BLM Principal Meridians and Base Lines Map: https://www.blm.gov/sites/default/files/meridianmap09_1.jpg.
Meridian - Lands described by the PLSS that share a common initial point, principal meridian, and base line. In Alaska there are 5 meridians: Copper River Meridian, Fairbanks Meridian, Kateel River Meridian, Seward Meridian, and Umiat Meridian.
Public Land Survey System (PLSS) - Both a method of subdividing lands and a method of describing lands. Additional information is available from the BLM at: https://mlrs.blm.gov/s/article/PLSS-Information and https://www.blm.gov/sites/blm.gov/files/Management%20of%20Land%20Boundaries.pdf
Principal Meridian - A north-south (true north) line that is established at the initial point (origin) of a meridian. See BLM Principal Meridians and Base Lines Map: https://www.blm.gov/sites/default/files/meridianmap09_1.jpg.
Selection - An application for lands submitted to the Department of the Interior in fulfillment of a land grant. A selection is valid if the lands applied for were vacant, unappropriated, and unreserved at the time of selection.
Top-file - A selection application which is not yet valid, but may become valid. A selection is top-filed if the lands applied for were not vacant, were occupied, or were reserved at the time of selection.
Township - An approximately 6-mile x 6-mile square unit of land described by PLSS; 36 Sections comprise a township. May also refer to the number of township lines north or south of a given meridian's base line.