There are a **lot** of definitions in Euclid's Elements. It'll be very hard to go through every one of them. To actually read Euclid's Elements yourself, you can go to http://aleph0.clarku.edu/~djoyce/java/elements/bookI/bookI.html. The definitions in the first book are largely self explanatory (a straight line is defined as the shortest line between 2 points, for example)

In addition to plane geometry (2d geometry) in book 11, Euclid also goes to define solid objects such as spheres, cones, parallelopipeds and pyramids. As such, the book also suggests the use of 3d geometry. This is elaborated upon further in books 12 and 13.

**Further Reading**

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