An uptrend is a continuous upward movement in a stock's price.
An uptrend is an upward movement over a few increments of time (whatever time increment being used), where the successive numbers being compared continue to increase. The parameters being compared might be just peaks, just troughs, closing prices, or averages, but formally it is defined as increased in successive peaks and troughs both.
A security purchased at the beginning of an uptrend will increase in value until the trend reverses (downtrend), and the uptrend is officially over when a successive peak or trough fails to beat the value before it. Some traders use that point as the selling signal to get out, and to look for another uptrend in a different security.
Most traders use other means of technical analysis in combination to determine when to buy or sell, such as the MACD.