Secular trend means -
Long term or secular trends 

The term secular trend implies changes in the occurrence of disease over a long period of time, generally several years or decades.
There may be progressive increase or decrease in the occurrence of disease.

Examples -

CHD, lung cancer, and DM have shown a consistent upward trend in developed countries.
TB, typhoid fever, diphtheria and polio have shown a declined trend in developed countries.
Seasonal changes
Periodical changes
Religion changes
Long term changes