License compatibility
Whether code under two different licenses can legally be combined into one program — and in which direction.
Two licenses are compatible if you can satisfy both at once. Permissive licenses are easy: MIT code can be folded into a GPL program, since the GPL's terms don't violate MIT's. The reverse doesn't work, which is why compatibility is always described as a direction rather than a property.
The famous conflicts are small details with large consequences. Apache 2.0 has a patent clause that GPLv2 regards as an additional restriction, so the two don't mix, whereas GPLv3 was drafted specifically to accept Apache. Sun chose the CDDL for ZFS, which is why the filesystem still isn't in the Linux kernel. Tools that read SPDX identifiers can flag the clashes, but interpreting them is still a lawyer's job.