Euclid's Subject is GEOMETRY, which is a branch of mathematics that studies the properties and relationships of points, lines, angles, surfaces, and solids. Euclid was an ancient Greek mathematician who is often referred to as the 'Father of GEOMETRY'. He wrote a book called Elements, which is considered one of the most influential works in the history of mathematics. Euclid's Subject is the study of shapes, sizes, and relative positions of figures and their properties. It is used in many fields such as architecture, engineering, and physics.