What is the strings on a musical instrument called?

"E" string, and "A", and "D", "G" string.

History of the strings:

Prior to the mid-20th century, double bass strings were usually made of gut, but since that time, steel strings have largely replaced gut strings, because steel strings hold their pitch better and yield more volume when played with the bow.[6] Gut strings are also more vulnerable to changes of humidity and temperature, and they break much more easily than steel strings. Gut strings are nowadays mostly used by bassists who perform in baroque ensembles, rockabilly bands, traditional blues bands, and bluegrass bands [7] Gut strings create the dark, "thumpy" sound heard on 1940s and 1950s recordings. The late Jeff Sarli, a blues upright bassist, stated that "starting in the 1950s, they began to reset the necks on basses for steel strings", and double bass players switched from gut strings to steel strings.[8] Rockabilly and bluegrass bassists also prefer gut because it is much easier to perform the "slapping" upright bass style (in which the strings are percussively slapped and clicked against the fingerboard) with gut strings than with steel strings. (For more information on slapping, see the sections below on Modern playing styles, Double bass in bluegrass music, Double bass in jazz, and Double bass in popular music). The change from gut to steel has also affected the instrument's playing technique over the last hundred years, because playing with steel strings allows the strings to be set up closer to the fingerboard, and, additionally, steel strings can be played in higher positions on the lower strings and still produce clear tone. The classic 19th century Franz Simandl method does not utilize the low E string in higher positions because with older gut strings set up high over the fingerboard, the tone was not clear in these higher positions. However, with modern steel strings, bassists can play with clear tone in higher positions on the low E and A strings, particularly when modern lighter-gauge, lower-tension steel strings (e.g., Corelli/Savarez strings) are used.