A secular trend is a long-term phenomenon, whereas a cyclical trend is short-term and will eventually reverse.
Secular trends are usually based on structural, underlying changes that mean something has altered that will affect asset values in the long term. Cyclical trends relate to the state of the economy: when it rises, so will the asset, and vice versa.
For example, MoneyWeek often says commodities are in a secular bull market: this is because the industrialisation of China has changed the long-term influences on the sector. Housing is a cyclical market: when interest rates are low and the economy is good, the sector performs strongly, but when rates rise and the economy weakens, it does badly.