an angle whose measure is between 90 and 180
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a point
A point.
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.
There are typically three words in geometry that are undefined. The first is "point." A point has no dimension, length, width, or thickness. The second is "line." A line has no thickness and goes on indefinitely in both directions. The third undefined term is "plane." A plane has no thickness and has no boundaries.
a plane