'“An overlap between a predecessor and a successor. For example, if a task can start when its predecessor is half finished, use a finish-to-start dependency with a lead time of 50 percent. Lead time is entered as a negative lag value.'
SOURCE; microsoft project glossary-projectlearning.net
A modification of a logical relationship which allows an acceleration of the successor task. For example, in a finish-to-start dependency with a 10 day lead, the successor activity can start 10 days before the predecessor has finished. See also lag.