JVMTI stands for the Java Virtual Machine Tools interface. Introduced in Java J2SE 1.5 (Tiger), this interface allows an a profiling agent, written in C, to gather diagnostic information about the JVM such as memory usage, method invocations, and time spend within methods.
The JVMTI replaces an experimental interface called the JVMPI (Java Virtual Machine Profiling Interface). They are very similar, both providing a C interface to the JVM, but the JVMTI improves on the JVMPI by providing more complete interface to the profiling agent.