Tennessee
Tennessee was admitted to the Union on June 1, 1796 as the 16th state. It was the first state created from territory under the jurisdiction of the United States federal government. Apart from the former Thirteen Colonies only Vermont and Kentucky predate Tennessee's statehood, and neither was ever a federal territory. The state boundaries, according to the Constitution of the State of Tennessee, Article I, Section 31, stated that the beginning point for identifying the boundary was the extreme height of the Stone Mountain, at the place where the line of Virginia intersects it, and basically ran the extreme heights of mountain chains through the Appalachian Mountains separating North Carolina from Tennessee past the Indian towns of Cowee and Old Chota, thence along the main ridge of the said mountain (Unicoi Mountain) to the southern boundary of the state; all the territory, lands and waters lying west of said line are included in the boundaries and limits of the newly formed state of Tennessee. Part of the provision also stated that the limits and jurisdiction of the state would include future land acquisition, referencing possible land trade with other states, or the acquisition of territory from west of the Mississippi River.

Q: When Tennessee's border was first defined, territory west of which river was considered for future annexation?
Mississippi River