Barometric pressure is defined as the force per unit area due to earth's atmosphere.
TIDBIT: There is a distinct difference between the actual barometric pressure and sea level barometric pressure. The actual barometric pressure is the true amount of pressure above a fixed point. Sea level barometric pressure makes the assumption that an observing station is actually at sea level. This results in the actual barometric pressure being lower than the sea level barometric pressure if the station has an elevation above sea level. For example, when the NWS reports a barometric pressure of 1020 mb for Denver, Colorado we know that the actual barometric pressure is much lower than this because of Denver's high altitude. Pressure is measured using a sea level reference in order to standardize pressure. With standardization, the regions with high and low pressure can be plotted. Otherwise, high elevation regions would always have low pressure.
PERPLEXION: The average sea level barometric pressure is defined as 1013.25 mb (29.92" of mercury). Most of the time on land areas over the United States the sea level barometric pressure will be higher than average. Why? I have not been able to find how the average barometric pressure is found. If someone knows let me know. I suspect that the average tends to be lower than what I observe on a regular basis since the tropics take up a larger area of the earth's surface and the oceans take up 70% of earth's surface. Perhaps pressure tends to be lower over tropical areas and ocean areas than over higher latitude and land areas. I usually only see the pressure go below 1013 mb in the United States when a significant area of low pressure has formed. But those low pressures cover only a small area and and do not occur often enough to give an average pressure each day of 1013 mb.