A trend channel is simply sideways trending in a stock. It can be viewed as the technical range between support and resistance levels that a stock price has traded in for a specific period of time via day trading system, option trading systems, and/or future trading systems.
There are several channels seen in day trading and stock trading throughout a given period of time, meaning this is a common occurence. A breakout of a technical channel can bee seen as bullish (upward breakout) or bearish (downward breakout) signals.
Here's a shot of a trend channel in NKE pulled strate for the Live Trading Room.
Related terms: Dow theory, uptrend, downtrend, primary trend, secondary trend, and minor (short term) trend.