2. Also a full orbit around a planet (for satellites/moons) or star (for planets).
The origin is interesting: Copernicus published a book in 1543 (apparently it's title is contraversial) Called something to the effect of: "On the Revolution of Heavenly Spheres". The Heavenly spheres is the part in question: some say it's Heavenly Orbs, celestial spheres, Heavenly bodies... but I went with the most common theory.
This book caused such an uprising, that the word revolution gained it's new and now common meeting.
2. It takes 365.24 days for the Earth to make a full revolution around the sun (according to the atomic clock), which is why our leap year adds another day every four years-- to make it reach 365.25. Because these numbers don't match exactly, our years are actually inaccurate, moreso as time goes on.