A gap fill is when a stock fills a previously created bearish or bullish gap. There are three types of gaps; Breakaway Gaps, Measuring (or Running) Gaps, and Exhaustion Gaps. Breakaway Gaps may possibly never fill. Measuring Gaps have a fairly good chance of filling. Exhaustion Gaps fill 100% of the time.
There are also occasions, mainly seen in a Day Trading System, that result in an intraday gap that is filled quickly. This does not fall under any of the three gap types mentioned above. It is simply a gap.
Here is a visual example of a gap being filled in ADBE:
The gray oval indicates the bullish gap. As you can see, the gap is followed by heavy downside action resulting in a fill of the bullish gap. These gaps often times act as magnets; pulling the price back to the starting point of the gap fairly quickly. That said, not all gaps filled as insinuated above.
Related terms used in trading systems and day trading systems: Bullish Gap, Bearish Gap, Breakaway Gap, Measuring Gap, Exhaustion Gap