A definition for Eavesdropping
A good definition of eavesdropping needs to be quite broad.
130+ years ago eavesdropping was a self-describing word. Standing outside someone's home to physically hear interior conversations "leaking" through the eaves was the primary method snoop's used to hear private conversations.
Today's snoops have more intelligence to collect, and more collection techniques available. In addition to private conversations, they are interested in: phone calls, computer data, e-mails, video teleconferencing, fax transmissions, off-site meetings, executive home offices, conversations held in cars and airplanes, etc.
The collection tools now include dozens of electronic interception techniques; from cheap FM radio transmitters (room bugs) and cell phone bugs to satellites with incredible optical capabilities. Tomorrow's snoops will be mind reading, and perhaps even "future peeping."
So, with all that in mind here is our definition of eavesdropping (at least for today)...
"Unauthorized, real-time access to intelligence."
(Just drop the term "real-time" for a definition of snooping.)