The sea state is defined by the characteristics of the waves at a particular location in a specific moment. To better define it, we have to identify the wave height (meters), being the same assigned by the average value of the trough of the wave crest height and the wave period (seconds), which is the average time interval between the passage of successive wave crests.

In this description we can still distinguish ripple from wave. A ripple is generated by the wind on its own place, it presents an irregular appearance with irregular ridges and waves from several directions in the course of the wind. A swell is generated by the spread of the ripple caused by the wind on a distant place, it presents a more regular appearance with rounded crests and the wave has a well-defined direction.

So we can characterize the sea as a large system of waves, each with its height, period and direction.