In telecommunication, a mixer is a nonlinear circuit or device that accepts as its input two different frequencies and presents at its output (a) a signal equal in frequency to the sum of the frequencies of the input signals, (b) a signal equal in frequency to the difference between the frequencies of the input signals, and, if they are not filtered out, (c) the original input frequencies.
Source: from Federal Standard 1037C
In beverage preparation, a mixer is a drink (usually non-alcoholic) that is mixed with another in the hope that the result is more palatable.
In dance, a mixer is a type of dance that encourages the participants to actively pursue new dance partners. A secondary result of this "ice breaking" maneuver is the generally increased level of enjoyment as the result of doing something unexpected.