The difference between leakage and seepage is as follows. All soils will adsorb and transmit water. I would classify seepage as water that is moving through the soil due to this natural ability of water to pass through the soil. I would classify a leak as water that is running out bypassing the soil. This can be either because the pond lining cracked and the water is running down to a more porous layer of soil, or there was a small pipe of soil that runs through the liner to a more porous layer. Water running through these will can make the hole open wider and wider. Sometimes you may have a seam of gravel or sand that runs through your pond lining that will not erode and yet leaks quite quickly. I would say if the soil in such an area becomes more porous due to water flow or was much more porous than the overall liner then I would call it a leak. Also water moving through any rock cracks would be called a leak (if you have ledge in your pond).

Thus seepage is water moving through a uniform pond soil lining is seepage. You should have a uniform lining. While water flowing through an unusually porous spot in your liner would be a leak. Like many things there are gradations between the two.