Landing footprint
A landing footprint, also called a landing ellipse, is the area of uncertainty of a spacecraft's landing zone on an astronomical body. After atmospheric entry, the landing point of a non-powered spacecraft will depend upon entry angle, entry mass, atmospheric conditions, and drag. It is therefore infeasible to calculate the spacecraft's landing point with absolute precision. By simulating entry under varying conditions, an ellipse can be calculated, resembling a footprint. The size of the ellipse represents the degree of uncertainty for a given confidence interval. References

Further reading 
 

Spaceflight concepts