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The terrestrial planets are just the four inner planets. Terrestrial is also just another fancy name for "rocky".
These planets include Mercury, Venus, Earth, and Mars. Like their name, they all have one thing in common: their surfaces are all solid and made of rock. Sure, atmospheres count as gas, but if you were suddenly dropped from somewhere above each planet, you would hit ground (though painfully) instead of passing through layers upon layers of gas. If you were on a gas planet, however, you'd probably become tired after swimming through all those gases you encountered before hitting the only solid, the core.