Points, lines, planes, and other geometric figures that exist only in an abstract way within your mind. Just like everything else.

Tesseract, 4th Dimension Made Easy

Carl Sagan. Duration_9_29

Specifying Planes

Khan Academy. Duration_4_11

Basic Geometry_Language and Labels

Khan Academy. Duration_12_57

Hidden Dimensions: Exploring Hyperspace

Brian Greene. Duration_1_27_15

Imagining the Tenth Dimension

Having trouble imagining more than 3 dimensions? What about the 10 or 11 promised by string theory? You wouldn't be alone. To help us, some Canadian guy (Rob Bryanton) put together this animation. Duration_12_45

Geometry 4(A) Distinguish between undefined terms, definitions, postulates, conjectures, and theorems