As a former prosecutor and now criminal defense lawyer, people ask me all the time, what is eavesdropping? Or when is eavesdropping illegal? And the legal definition of eavesdropping here in California is when you listen in on a private conversation without the knowledge or consent of the parties and you use either a recording device to record the conversation or an amplifying device to help you hear the conversation.
Now, I'll give you an example. Suppose you're in the bathroom stall at work, and two co-workers come in and they start talking about some confidential matter. They don't realize you're in the bathroom stall, and you just quietly overhear their conversation. That is not eavesdropping. That's at least not illegal eavesdropping as the law defines it because you're not using any sort of device to amplify or record the conversation.
But suppose you take out your iPhone and you decide, I'm going to take advantage of this opportunity. I'm going to record what they're saying. And you record their conversation. Then it becomes eavesdropping, and that would be illegal.
Eavesdropping laws in California can be found in Penal Code 632.