If you want the technical definition, it is that the LEO, under the totality of the circumstances then present, has reasonable cause to believe that it is more likely than not that contraband or evidence of a crime is located in the area in question. Now, the officer is allowed to take into consideration his or her training and previous experience when interpreting the facts at hand. "Hunches" don't count, you have to be able to articulate to the court specific reasons why you felt you had PC. For instance, I stop a car for speeding. It's one in the morning in an area known for drug trafficking. I walk up to the car, the driver hurriedly gets out to meet me. He is nervous and sweating, even though it's 50 degrees outside. For my safety, I ask him to get back in the car (yes, I can order him back into the car even if all I have is a traffic infraction- the U.S. Supreme Court ruled a while ago that officers have the right to control drivers and passengers on normal traffic stops for safety reasons. I do not have to be able to articulate specific reasons why I thought this particular person was a threat, I can tell them to do basic things like get back into or get out of the car simply for my safety). I walk up to the car behind him, see him stuff something under the seat. I look in the window and see some small rubber balloons lying on the floorboard, tightly tied with small pieces of some substance inside. Since I've worked dope, I know based on my training and experience that drug dealers frequently package drugs, especially heroin, in such balloons. Because I have that training and experience, I now have PC to search the car. An officer who doesn't have the same training and experience and cannot articulate the drug packaging may not have PC at this point, even though they are suspicious. However, his nervousness and his stuffing something under the seat definitely give me reasonable suspicion to do a 'pat-down' search of the area under the seat for a weapon. In fact, as soon as he reaches under the seat he's probably going to get a gun screwed in his ear.
The problem lies not in the technical definition of probable cause, but in the fact that PC is something that the officer has to decide he or she has based on the totality of the circumstances. Every situation is different, and every officer has slightly different training and experience. That decision about having PC will then be second-guessed by attorneys and judges who have the benefit of evaluating the situation after the fact and having all the information. It's impossible to make one concrete, hard and fast rule that can quickly and easily be applied in every situation that an officer could possibly face. If you think you're frustrated with PC or search and seizure law, try working the streets for a few years. Just when you've got it somewhat licked, the courts come out with a new ruling that changes everything.