The independent variable is the condition that is a manipulated by an experimenter. (You can think of the independent variable as being independent of the actions of those taking part in the experiment; it is controlled by the experimenter.) In the case of the Latane and Darley experiment, the independent variable was the number of people present, which was manipulated by the experimenters.
Independent variable is the variable in a study whose changes are the basis for predictions because they are not dependent upon changes in any other variable in the study. The most familiar example of an independent variable is an experimental variable.