zombie process

An entry in the process table kept for a process that has already exited, held there until its parent collects its exit status. ps shows Z in the STAT column, sometimes <defunct> in COMMAND. Nothing is executing: no memory held, no open files, no CPU consumed. So no signal reaches it, -9 included — you cannot kill what has already died. The fault lies with the parent, which is not reaping its children. Fix or restart the parent and the zombies go at once; if the parent dies, PID 1 adopts its children and does the reaping in its place. A handful in top's zombie count is the normal life of a machine. Thousands are a leak of table entries, and the supply is finite: /proc/sys/kernel/pid_max is 4194304 on Ubuntu 24.04.