There are different geometries but in the most intuitive one, Euclidean Geometry, the undefined "objects" are point, line and plane while incidence, congruence and being between are undefined concepts.
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point,line,and plane
Point, line and plane.
The three undefined terms are the point,the line and the plane.
A point, line and plane
the "building blocks" of geometry(which are the point,line and plane)are so-called undefined because, technically,they can't be described without the aid of words which are undefined themselves.