Intersat code was a type of inter-stellar communications code used during the 21st century. By the second half of the century the code was outdated and rarely used.
The inhabitants of Terra 10 colonized that planet during the era of intersat code and still relied on it for communication nearly 200 years later. (TAS: "The Terratin Incident")
This form of communication was described in the first edition of the Star Trek Concordance as "interstat code."