What is a defining characteristic of plane figures?

A defining characteristic of plane figures is that they possess no thickness. This means that plane figures exist only in two dimensions, which are length and width. Examples of plane figures include squares, circles, and rectangles, all of which lie flat in a single plane without any vertical dimension. This concept is fundamental in geometry, as the absence of thickness allows for the interpretation of these shapes on a flat surface, such as paper or a computer screen.

The other options do not accurately characterize plane figures. Only having one straight edge would not apply to most plane figures, as many, like rectangles and polygons, can have multiple edges. The concept of having no size is misleading since plane figures actually have measurable dimensions, albeit only in two dimensions. Lastly, stating that they have no measurement is incorrect, as plane figures can certainly be measured in terms of area and perimeter. Thus, the characteristic of having no thickness distinctly identifies plane figures in the study of geometry.