Illustrated Glossary of Organic Chemistry

Steric strain (van der Waals strain): An increase in molecular potential energy (strain) caused when atoms or groups separated by at least four covalent bonds are forced closer than their van der Waals radii allow.

In the eclipsed conformation shown here, hexane's terminal methyl groups experience steric strain, because these methyl groups are forced closer than their van der Waals radii allow. In other words, the methyl groups are forced to partially share the same space. The resultant electron repulsion causes an increase in molecular potential energy (strain). Explore this issue further with your molecular model kit.