The Federation-Cardassian border was the shared border between the United Federation of Planets and the Cardassian Union. During the early 2370s, a Demilitarized Zone was established between a section of Federation and Cardassian space.
With the Dominion takeover of Cardassia, the Cardassian border effectively became the Dominion border; the Cardassians retained control in name only.
Over the course of 2375, numerous Starfleet crew were captured by Cardassians and taken across the border to the rasgălor ("outworld" in Cardăsda, a term used for any world located far from the Cardassia system) of Lessek. As it turned out, these Cardassians were rebels who sought the Starfleet crewmembers' aid in their uprising, and the Starfleet contingent agreed to the chance to create havoc behind Dominion lines. (Star Trek: Sigils and Unions--The Thirteenth Order)
Natives of some Federation worlds near the border or DMZ have been observed to speak a dialect of Federation Standard that is heavily influenced by the vocabulary and syntax of the Cardăsda language--though to mention the origin of these "cardasdanisms" is highly offensive to some. (The Thirteenth Order)