A trend is a movement in a certain direction that can be observed by data that can be graphed. Actually, you can have a trend without a graph, but if you Could graph it somehow it would be a trend. It is something that has actually happened. A tendency is a predisposition for something, or a bias that makes one thing more likely than another. A tennis player can have a tendency to hit the ball long on the backhand. A tendency is not something that has happened. It is a likelihood or increased chance that something will happen in a certain way.