Something that can't be determined or defined, basically.
"Defined items" are defined in terms of "undefined terms".
A point.
Point
define since un means not
Undefined is defined. Define means to set the limits, explain. So, undefine means not to set limits or not to explain.
Line, plane
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I am prepared to define any term in geometry. If there is some term for which you would like to have a definition, just ask.
They are undefined because they can't be described without using words that are themselves undefined. For example a point has no dimensions. It is a location, or position. But if you describe it that way you have to define location or position. You can't define backwards infinitely, so we accept the meanings of these things as intuitively clear without definitions because they are the building blocks of geometry and are needed to define much more complex terms.
undefined, zero, negative, and positive are the 4 types of slopes
When you are a tour guide and you try to give a tour of someone elses Igloo you see the word undefined, which means you can't do that.
It is a very basic concept which cannot be defined. Undefined terms are used to define other concepts. In Euclidean geometry, for example, point, line and plane are not defined.