Definition of a trendline
A line that is drawn over pivot highs or under pivot lows to show the prevailing direction of price. Trendlines are a visual representation of support and resistance in any time frame. (From Investopedia)
To add to this description a properly constructed trendline should be drawn through 3 or more swing high or swing low points. The higher the timeframe the trendline is constructed on the higher the integrity of the trendline.... for example if the trendline is drawn on a daily chart, the trendline has more significance than a trendline drawn on a 5 minute chart. Price is far more likely to respect the support and resistance levels created by trendlines on the longer timeframes.
In the same way... the more times price has tested a specific support or resistance level the more significant this level becomes.