Mergers
From #openttdcoop wiki
Mainline Mergers are those places associated with BBHs where the lines of two (or more) incoming directions merge into one outgoing direction. There are various methods to build these. A brief overview can be found in this blog post, and many examples can be seen in the Junctionary.
The size of mergers is usually in the form x+y->z, which means that x lines from one direction and y from another direction are merged to z outgoing lines. If only one number is given (like "LL merge" for two lines), it is the number of outgoing lines.