What is Bull Trap?

Legal Definition
In economics, a Bull trap is an inaccurate signal that shows a decreasing trend in a stock or index has reversed and is now heading upwards, when in fact, the security will continue to decline.

It is seen as a trap because the bullish investor purchases the stock, thinking it will increase in value, but is trapped with a poor performing stock whose value is still falling.
-- Wikipedia
Legal Definition
Any technically unconfirmed move to the upside that encourages investors to be bullish. Usually precedes important declines and often fools those who do not wait form confirmation by other indicators.