The envelope of gases surrounding the earth. Mixture all types of gases.

The term Atmosphere is also used to describe the conditions in or about an occupied room. Examples the dark atmosphere of the museum, Most of the Paris hotels have very pleasant atmosphere.

The ability of the solar wind to blow away a planet's atmosphere has very little to do with gravity. On Earth, the magnetic fields generated by the planet's iron core creates the magenetosphere. Most of the solar wind consists of ions, which are charged particles; the Earth's magnetic field deflects these, so that the bulk of the solar wind is deflected and flows around our atmosphere without disturbing it much.

On Venus, the outermost layer of the atmosphere is a very thick, electrically charged ionosphere. It acts similarly to a magnetic field, causing the solar wind to deflect and not remove Venus' atmosphere.

The probes we have sent to Mars indicate that, while that planet apparently had some magnetic field around it in the past, it does not have one now. Consequently there is nothing the deflect the solar wind; many scientists now believe that Mars had an atmosphere, but lost it when the magentic field failed.

There is an excellent Nova video called "Magnetic Storm" about this; if you are interested in learning more about planetary magnetic fields, you can watch it here.

Maybe it is because, starting with Venus, it is at the right proximity to the sun to hang on to its atmospheric gases, to not have them "blown" away.  Venus is the same size as Earth, so the gravitational attraction should be the same and Venus should have the same gravitational attraction to be able to hold onto its atmosphere.  But then you have Mars, further out than either of them, with very little atmosphere.