Secular trends in the economic/market/investment sense are trends that aren't seasonal or cyclical. They are long term trends not affected by current or present economic factors.

“Secular trend” is used a lot in health and science. It basically means long-term trend, or the trend on the longest time scale.

It means a long term trend that is not effected nor related to the cyclical and seasonal trends.

"Secular" is derived from the Latin "saeculum" and it can refer to long-term trends, trends that persist for a long time. A "saeculum" was a long period of time. A century is a specific number of years. The length of a saeculum was variable and based on human life. A saeculum ended when the there were no longer any survivors from the beginning of the saeculum. In investing, a "secular bear market" means a bear market that continues for a long time.