Experiments   
 Experiment #7 Sample D Flip-Flop Waveform
This is the simulation of the positive edge-triggered D flip-flop. As you can see, Q assumes the value of data only at the positive edge of the clock. This means that Q can change value at most 1 time per clock cycle. This can be clearly seen during the interval from 180ns to 250ns. This is what is meant by edge triggered.

D Flip-Flop Waveform