Messaging Extraterrestrial Intelligence: deliberately transmitting to other stars rather than listening for transmissions from them, and the unresolved argument about whether it should be attempted at all. Also called active SETI. It is the half of the question that SETI does not cover, and the two have different risk profiles: listening is free, transmitting cannot be recalled.
The record
The deliberate transmissions are few and almost all symbolic. The Arecibo Message of 1974 is the first and the most powerful. The Cosmic Call transmissions went out from the Yevpatoria dish in Ukraine in 1999 and 2003, the Teen Age Message in 2001, A Message From Earth in 2008, Lone Signal in 2013, and Sonar Calling GJ 273b toward Luyten’s Star in 2017 and 2018. None was aimed at a target that could reply within a human lifetime except the last, whose target is twelve light-years away.
The argument
In 2015 a public statement signed by astronomers, technologists and science fiction writers called for an international discussion before any further transmission, on the ground that a decision affecting the whole planet was being taken by individual teams. The objection is that a targeted coherent beam is not the same thing as the incoherent leakage from radar and broadcast that already escapes, so the common reply that we have been announcing ourselves for a century understates the change. The counter-objection is that the sample size is one, that no observation supports the hostile prior, and that a policy of permanent silence cannot be verified or enforced. The Dark Forest Hypothesis is the strongest form of the case against, and its weakness is the same one it has everywhere: it needs unanimity, and a single transmitter ends it.
Connections
SETI · Arecibo Message · Dark Forest Hypothesis · Wow! Signal · Fermi Paradox · The Three-Body Problem