A splitter; a device used to split a single jack into two.
2006: Edward C. Baig, Macs For Dummies (9th ed.), p. 137The answer is a small, inexpensive, plastic doodad called a splitter (or Y-jack), which as its name suggests splits a single phone jack into two. So now you can plug your computer and phone into the same wall jack
2007: Rick Steves & Pat O'Connor, Rick Steves' Ireland 2008, p. 14If you bring along your own pair of headphones and a Y-jack, two people can share one audioguide and save.
