Secular variation

Long-term non-periodic variation

The secular variation of a time series is its long-term, non-periodic variation. Whether a variation is perceived as secular or not depends on the available timescale: a variation that is secular over a timescale of centuries may be a segment of what is, over a timescale of millions of years, a periodic variation. Natural quantities often have both periodic and secular variations. Secular variation is sometimes called secular trend or secular drift when the emphasis is on a linear long-term trend.

This keyword could refer to multiple things. Here are some suggestions: