Write three undefined terms of geometry.

Hint: In every branch of Mathematics, we use some undefined and simple terms to define much more complex terms. Every branch of mathematics has some undefined terms.

Complete step by step answer:
In the Euclidean Geometry, we use three undefined terms to define many terms that we use in geometry. In geometry there are three undefined terms using which the whole geometry has been developed over years. Those three undefined terms are
1. Point
2. Line
3. Plane
Now, let us go through these undefined terms one by one.
1. Point
$\Rightarrow $ A point indicates the location in a space of any dimension.
$\Rightarrow $ It does not have any length, width, or height.
$\Rightarrow $ We represent a point using the symbol dot.
$\Rightarrow $ We denote a point using capital letters, For Example: A, B, C etc.

2. Line
$\Rightarrow $ A line is of infinite length.
$\Rightarrow $ It does not have any thickness or height.
$\Rightarrow $ A line is assumed to be straight joining all the collinear points between two points and extending to infinity.
$\Rightarrow $ We denote a line using small alphabets or using two points which lie on the line. For example: line $l$, line $\text{AB}$.

3. Plane
$\Rightarrow $ A plane has infinite length and infinite width.
$\Rightarrow $ It does not have any height.
$\Rightarrow $ A plane has two dimensions.
$\Rightarrow $ We represented a plane by small letters or using three points lying on the plane. For example: plane $a$, plane $\text{ABC}$.


Note: Just because these terms are undefined does not mean that these words are not valid or not useful. We cannot define these terms but we can describe them and we use them for defining many terms required.