A secular trend is one that will is sustained (or is expected to be sustained) over the long term. The term is most often used to distinguish underlying long-term trends from seasonal variations and the effects of economic cycles.
There are many techniques for separating out secular trends from seasonal, and other, variations in historical data. These range from simple year on year comparisons to complex econometric models.
Secular trends are obviously what matter to investors, but investors need to look beyond historical trends, or even short term forecasts, and consider the long term sustainability of trends.