Final answer:
The new substance discovered by the scientist, where the atoms are close but the volume is indefinite, is best defined as a liquid.
Step-by-step explanation:
The substance discovered by the scientist is best defined as a Liquid. In a liquid, the atoms are close together, but they have the ability to flow and move around each other, which results in an indefinite volume. Unlike solids, liquids don't hold to a rigid shape, but instead takes the shape of their container. This characteristic of liquids allows the volume to be indefinite as the shape can change based on the container it is in.
Learn more about States of Matter