Java thread lifecycle refers to the different states that a thread can be in during its execution.

1. New: When a thread is first created, it is in a "new" state. At this point, the thread has been created but has not yet started running.

2. Runnable: A thread is said to be in the "runnable" state when it is eligible to run but is not currently running. In other words, it is waiting for a CPU to be allocated to it.

3. Running: When the CPU is allocated to the thread, it enters the "running" state. This is the state in which the thread is actively executing its code.

4. Blocked/Waiting: Sometimes, a thread may need to wait for some external event to occur (e.g. waiting for user input). In these cases, the thread enters a "blocked" or "waiting" state. It does not compete for CPU time and does not execute any code until the event it is waiting for occurs.

5. Terminated: When a thread completes its execution or is interrupted, it enters the "terminated" state. This is the final state of the thread's life cycle.

It is important to note that threads can transition between these states as they execute. For example, a running thread may become blocked while waiting for user input, and then return to the runnable state once the input is received. Understanding the different states of the Java thread lifecycle is key to building complex multi-threaded applications.