A bear market is when the price of an asset class, or market, declines at least 20%, over at least a two-month period.
A market trend is a tendency of financial markets to move in a particular direction over time. These trends are classified as secular for long time frames, primary for medium time frames, and secondary for short time frames. Traders identify market trends using technical analysis, a framework which characterizes market trends as predictable price tendencies within the market when price reaches support and resistance levels, varying over time.
Strictly, a trend can only be determined in hindsight, since at any time prices in the future are not known, although derivatives such as futures and options can give a clue about expectations.
The terms bull market and bear market describe upward and downward market trends, respectively, and can be used to describe either the market as a whole or specific sectors and securities. The names perhaps correspond to the fact that a bull rampages forward, while a bear protects itself and (often) retreats.