Backward binary compatibility (or downward compatibility) - an ability of clients built with an old version of library API to run on a new one (wiki).
Upward binary compatibility (or forward compatibility) - an ability of clients built with a new version of library API to run on old one (wiki).
The general Sun's document about JDK Incompatibilities in J2SE 5.0 since 1.4.2 (and Java SE 6 compatibility with J2SE 5.0 too) describes the compatibility of JDK as following:
JDK 5.0 is upwards binary-compatible with Java 2 SDK, v1.4.2 except for the incompatibilities listed below. This means that, except for the noted incompatibilities, class files built with version 1.4.2 compilers will run correctly in JDK 5.0.
I suppose that documentation writers have mixed up terms "upward" and "backward" compatibility in this sentence. They describe a "backward" compatibility, but call this feature as "upward" compatibility.
Is this a typo, mistake or intended term here? Is JDK "upward" or "backward" compatible?
Note that for something to be backwards compatible there must be a counterpart that is forwards compatible (either intentionally or unintentionally). For example: are the DVD readers backwards compatible with CD's or are the CD's forward compatible with DVD readers?
In this case, it depends if you look at the compiler (or the bytecode it generates) or the virtual machine.
The compiler is not backwards compatible because bytecode generated with Java5 JDK won't run in Java 1.4 jvm (unless compiled with the -target 1.4 flag). But the JVM is backwards compatible, as it can run older bytecodes.
So I guess they chose to consider the compatibility from the point of view of javac (as it is the part specific to the JDK), meaning that the bytecode generated can be run in future releases of the jvm (that is more related to the JRE, but also bundled in the JDK).
In brief, we can say:
(And it also serves as a lesson that should be learnt long ago: the people writing the compilers are usually right, and we the people using them wrong xD)
By the way, doesn't it make more sense to pair backward/forward and downward/upward rather than mixing them up?