An object of indeterminate size marking a
location in space

Point

Explanation:

An object of indeterminate size which marks a location in space is called a point.

A point in geometry has no dimension unlike a line which is one dimensional. In essence, like other bodies, we cannot specify any of the width, length or height of a point.

On the other hand, a line is a connects two or more points and it is one dimensional. They usually have lengths.

Therefore, an object of indeterminate size which marks a location in space is a point.  

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.