Buying more of a stock you already hold after the price has fallen, which lowers your average cost per share. It is sound when the fall was a market swing and the business is unchanged, and it is how people lose the most money when it is not, because it concentrates more into a position that is already going wrong. The honest test is whether you would open the position today at this price knowing nothing about what you paid before.
See every term in the full PSE investing glossary.