This is the shortest "clinical" definition of Passive-Aggressive behavior.
Passive-aggressive behavior is a pattern of expressing your negative feelings in an indirect way, instead of openly addressing them.
People who are passive-aggressive appear to agree with the requests of others. They may even seem enthusiastic about them. But they don't perform a requested action on time or in a useful way, and may even work against it. In other words, they use nonverbal behavior to express anger or resentment that they can't express verbally. An example is showing up very late to a meeting that you didn't really want to attend and then making up excuses for your lateness that deflect attention from the real reason you were late.
Signs and symptoms of passive-aggressive behavior are things like:
Resentment and opposition to the demands of others
Complaining about feeling underappreciated or cheated
Procrastination
Stubbornness
Inefficiency
Memory lapses
Sullenness
Irritability
Cynical attitude
Although passive-aggressive behavior can be a feature of some psychiatric disorders, it isn't considered a mental illness. Researchers are studying how to classify habitual and problematic behaviors such as passive-aggressiveness.