An uptick is an incremental increase in the trading price of a security.
Uptick is a slight increase in the trading price of a security. The word comes from the "ticker price" of a stock, which used to be printed out on ticker tape from a printer connected to telecommunication lines which reported updates in trading information throughout the day. Now tickers run electronically across the bottom of television screens and so on.
Visually, also, an "uptick" is a fitting name for the movement of a line on a graph, especially since an axis might have tick marks on it. In terms of short selling, there is an Uptick Rule in effect, which states that short sales can only follow an uptick in a stock's price.
How much this rule works has been debated, and and there was no uptick rule between 2007-2010, but there is some logic behind it. Downtrends put selling pressure on a stock, and, conversely, selling pressure can exacerbate a downtrend.
If you stop short selling in a free-fall, hypothetically the price will not fall as fast. For these purposes, an uptick must be $0.01 or more, and must be in the purchase price and not just the Ask price.