OECD - Business tendency surveys
In time series analysis, a given time series can be decomposed into a trend component, a cyclical component, a seasonal component and an irregular component. An essential feature of the concept of trend is that it is smooth over periods that are long in relation to the unit of time for which the series is recorded. A trend is usually represented by some smooth mathematical function such as polynomials in the time variable or by moving averages.
Organization for Economic Cooperation and Development (OECD), "Business Tendency Surveys - A Handbook", OECD, Paris, 2003