“OpenJDK” is a trademarked term, per the OpenJDK Trademark Notice [1]. As such it should be used only as an adjective, and not as a noun. Phrases such as “the OpenJDK” could be replaced by the more correct, and much more verbose, “the OpenJDK JDK,” or “the open-source JDK,” but in most cases the context is sufficiently clear that we can just write “the JDK.”

[1] http://openjdk.java.net/legal/openjdk-trademark-notice.html