A BattleMech. Nova is the clan name - the Inner Sphere designation is Black Hawk.
Nova is a 50-ton OmniMech. It has been described as a "jack of all trades, master of none". Heavy 'Mech pilots keep joking about it: "when we use those, I need to watch out or I might step on them..."
In its primary configuration, it carries a whole lot of medium-extended-range lasers. This usually generates the thing that we in business call "excessive heat". Commonly seen alternate configuration includes extended-range PPCs, and it's not a bad choice. Another common configuration carries lasers, autocannons and machine guns for variety freaks.
Also, among the Clans, a Nova is a special type of star: A normal star of BattleMechs plus one point of Elementals, totalling 5 of both.