The phrase "impact basin" refers to a large depression or hollow on the surface of a planet or moon that was created by the impact of a meteoroid or asteroid. It is like a big dent caused by something hitting the surface really hard. Full definition
«By comparing the measured craters to the number and spatial distribution of large impact basins on Mercury, we found that they started to accumulate at about the same time, suggesting that the resetting of Mercury's surface was global and likely due to volcanism,» said lead author Dr. Simone Marchi, who has a joint appointment between two of NASA's Lunar Science Institutes, one at the SwRI in Boulder and another at the Lunar and Planetary Institute in Houston. (sciencedaily.com)
Combining these new estimates with the fact that there are even larger impact basins on the Moon and other planets, Schultz concludes that protoplanet - sized asteroids may have been common in the early solar system. (sciencedaily.com)
Below it the giant impact basin Hellas. (solarviews.com)