Defunct processes : SE 7.1 on AIX 4.1

I'm not sure if this is the correct group for this problem so please bear with me. When running an Informix query (SQL), a defunct process is being generated. If the session is killed by the Administrator or by a connection timeout, the defunct process continues to run and keeps the table locked from the previous instruction (UPDATE,INSERT,DELETE) until the Administrator kills the defunct process. My question is, why is a defunct process being initiated and what is its purpose? Informix SE V7.1 AIX 4.1.2