If you're at all familiar with legal TV shows or movies, you've probably heard the term "guilty beyond a reasonable doubt."
Many people who take their cases to civil court assume they need to prove their case beyond a reasonable doubt in order to get compensation. For that matter, many people who are selected to serve on civil juries make the same assumption and are surprised when they're instructed to find in favor of the party that has a "preponderance of the evidence" on their side.
"Preponderance" is not a word we use every day, but it's generally understood to mean "greater weight." That is, the jury is instructed to find in favor of the party that the weight of the evidence favors, even if only slightly.
Put another way, the winner in a civil case is the party that the jury decides is more likely than not to be correct, whether that likelihood is 51 percent, 99 percent or anywhere in between.
This difference in standards of proof is due to the difference in the powers of criminal and civil courts. A criminal court has the power to take the defendant's liberty (by sending them to prison) or even, in states with the death penalty, their life.
Clearly we need proof beyond reasonable doubt in order to take such measures. A civil court can only take the defendant's property, hence a lower standard of proof is appropriate.
This has been true for a long time, but probably not as long as you might think. The idea of a "preponderance of the evidence" only started to enter legal thought in the late 1700s, although the more general concept of balancing the scales of justice dates back all the way to the ancient world.
The modern standard wasn't codified and established in American courts until the mid-1800s.
What this means today is that many cases exist in the gap between "preponderance of the evidence" and "proof beyond reasonable doubt." Famously, in the O.J. Simpson case, Simpson was acquitted on criminal charges for his wife's murder but found in civil court to be responsible for her death.
Likewise, if someone is injured in an accident caused by criminal conduct (for instance, drunk driving), a defendant who is acquitted in criminal court can still be found responsible in civil court. This is why plaintiff's need a good lawyer to help protect their rights.