A lot of white papers explain momentum as a cross-sectional result. What this means is that future outperformance of a stock is predicated by its outperformance relative to its peers. (MCD vs GE) This is the standard way of doing it, ranking a universe of stocks based on ROC or some other measure.
The other one is time series momentum. This is not a new concept. The idea is that a securities past performance predicts its own future return. From this, auto-correlation is used. The main advantage is that time series momentum can measure and analyze all asset classes because it relies only on its own past price.
The reason I introduced these two different explanation for momentum is that both offer promising fundamental concepts that can be used to build robust strategies. I leave it to the reader to explore further.