Interesting to see in this thread how many different opinions there are as to what establishes a trend, and also how many traders there are asking for a definition or have doubts. Also, there is no mention of the basic textbook definition of what makes up a trend, even though trend lines are to be seen everywhere outlining trends. Anyway, here's my 2 cents.
There are basically 4 trends in a bull/bear market. The primary, intermediate, minor and sub-minor trends. The initial wave following a reversal of the primary trend, or that following a correction in an established trend, does not in itself qualify as a trend. The reason being that a trend requires an initial move followed by a corrective move of say one to two thirds of the initial move which is then followed by a move in the direction of the initial move that equals or exceeds the high/low of the initial move. This can be seen in the attached where the trend is established when the price movement reaches 'D', and not before. True, the move from e or C is in the same direction as that of A to B, but has not yet proved its strength to continue the A to B movement. So until D is reached or exceeded, there is no trend.
However, that is not to say that entry opportunities will not exist between C and D. But just for the purpose of defining a trend, D must be reached first.
I so often see trend lines that have been drawn, incorrectly IMHO, when the price has moved somewhat away from 'e', and hear of trades made around 'f' based on what is thought to be an established trend. That may work out. However, too often the price movement will move into a time consuming consolidation before continuing through D, or will result in the failure to continue as seen in the second chart.
In a variation of the one to two thirds retracement of the initial move, a narrow consolidation is often seen at the level of B. But once you have a breakout from that consolidation and D is established, you have a trend.
Actually, in the first chart you have both a retracement and a triangle pattern.
Based on this method of finding a trend, the 'it's all in hindsight' claims made by the guys on the other side of the fence won't hold up, as the move to D establishes a trend that can be seen in real time in any time frame and is the first opportunity to draw a 'correct'
trend line, IMHO.