You may hear meteorologists talking about occluded fronts, or occlusion. While it doesn't appear on our weather maps with its own symbol, an occluded front is merely a front that has been overtaken by another faster front.
Usually, cold fronts move faster than warm fronts, as the two wrap around the perimeter of their host low. Eventually, the cold front catches up, usually on the east side of the low. This occlusion is good news for the people in the low's path, because once these two fronts join, the low soon weakens and dies away.