People ask, “what is a trend?” One of the best discussions I have seen on the subject comes from top trend follower and one of my mentors Ed Seykota:

A trend is a general drift or tendency in a set of data. All measurements of trend involve taking a current reading and a historical reading and comparing them. If the current reading is higher than the historical reading, we have an up-trend. If lower, we have a down-trend. In the improbable event of an exact match, we have a sideways trend. The direction of the trend depends upon the method we use to perform the comparison. Real instruments fluctuate minute-to-minute, day-to-day and year-to-year. We have, therefore an enormous supply of historical points to use to determine trend. As such, we can determine as many instances of trend as we please, in any direction that we please. There is no such thing as the trend; there are countless trends, depending on the method we use to determine a trend. People typically pick a method for determining trend that fits with their current positions and/or view of the market…All methods of defining trends compare various combinations of historical price points. All trends are historical, none are in the present. There is no way to determine the current trend, or even define what current trend might mean; we can only determine historical trends. The only way to measure a now-trend (one entirely in the moment of now) would be to take two points, both in the now and compute their difference. Motion, velocity and trend do not exist in the now. They do not appear in snapshots. Trend does not exist in the now and the phrase, “the trend” has no inherent meaning. When we speak of trends, we are speaking, necessarily, from some or another view of history. There is no such thing as a current trend. When we speak of trends we are necessarily projecting our own definitions. With that in mind, we can proceed to examine ways to define, compute and use trends.

Well stated! In terms of “trend following”, in the words of Van K. Tharp:

Let’s break down the term trend following into its components. The first part is ‘trend’. Every trader needs a trend to make money. If you think about it, no matter what the technique, if there is not a trend after you buy, then you will not be able to sell at higher prices…’following’ is the next part of the term. We use this word because trend followers always wait for the trend to shift first, then ‘follow’ it.

My books and courses take these definitions to the level of detail actually needed to become a successful trend following trader.