1. A sudden or violent backward whipping motion.

2. An antagonistic reaction to a trend, development, or event: "As the backlash against divorce progressed, state legislatures ... called for a rollback of no-fault divorce laws and even for premarital waiting periods" (Walter Kirn).

3. A snarl formed in the part of a fishing line that is wound around the reel.

4. The play resulting from loose connections between gears or other mechanical elements.