A rally is a recovery in price after a period of decline.
It is a period in which the price of an asset sees sustained upward momentum.
Typically, a rally will arrive after a period in which prices have been flat or in a decline.
Rallies are caused by an increase in the number of people buying into a market. This increased demand leads to increases in price.
For example, a bear market rally is as a brief period of upward price momentum while the overall trend is a downtrend.