A treaty between the Federation and the Romulans.
The Treaty of Algeron is the agreement between the Federation and the Romulan Star Empire that governed the Romulan Neutral Zone in the twenty-fourth century. It reaffirmed that neither side may enter the Zone without notifying the other and includes a clause under which the Federation agreed not to develop or use cloaking technology.
The treaty is cited several times in The Next Generation, including in The Defector, when the Enterprise-D considers entering the Neutral Zone. In The Pegasus, Riker's former commanding officer, Admiral Erik Pressman, tries to recover the lost USS Pegasus, which had secretly tested a phasing cloaking device in 2358. Picard exposes the illegal experiment to the Romulans.
In Deep Space Nine, the Romulans lent the USS Defiant a cloaking device in 2371 in exchange for information about the Dominion, with a Romulan officer stationed aboard. The arrangement allowed the Defiant to use a cloak without breaking the treaty.