Area
Area[reg] gives the area of the two-dimensional region reg.
Area[{x1,…,xn},{s,smin,smax},{t,tmin,tmax}] gives the area of the parametrized surface whose Cartesian coordinates xi are functions of s and t.
Area[{x1,…,xn},{s,smin,smax},{t,tmin,tmax},chart] interprets the xi as coordinates in the specified coordinate chart.
Please visit the official Wolfram Language Reference for more details and examples on core symbols.