Step 1: A time series is decomposed into four components: secular trend (the smooth, long-term direction over many years), cyclical component (medium-term wave-like swings tied to business cycles), seasonal component (regular fluctuations within a single year tied to the calendar), and the irregular or random component.
Step 2: The phrase "general decline" describes a sustained, ongoing directional movement in sales measured over an extended span of years, not a fluctuation that repeats every year at the same calendar point and not an up-down swing over a multi-year cycle.
Step 3: A sustained long-term downward movement of this kind is precisely what is defined as the secular trend of a time series.
Step 4: Hence the general decline in sales of cotton clothes reflects the secular trend.
\[\boxed{\text{Secular trend}}\]