Viva, vive, and vivat are interjections used in the Romance languages. Viva in Spanish, Portuguese, and Italian, Vive in French, and Vivat in Latin are subjunctive forms of the verb "to live". Being the third-person, subjunctive present conjugation, the terms express a hope on the part of the speaker that another should live. Thus, they mean "(may) he/she/it live!" (the word "may" is implied by the subjunctive mood) and are usually translated to English as "long live".
They are often used to salute a person or non-personal entity: "Vive le Québec libre" (from Charles de Gaulle's Vive le Québec libre speech in Montreal), or "Viva il Duce!", the rough equivalent in Fascist Italy of the greeting, "Heil Hitler." Additionally, in monarchical times the king of France would be wished "Vive le Roi!" and the king of Italy "Viva il Re!", both meaning "May the king live!" or "Long live the king!".
The acclamation "Vivat!" is still used in British coronations when the sovereign is hailed while processing from the quire of Westminster Abbey towards the coronation theatre, fronting the altar. The shouts are delivered by the King or Queen's Scholars of Westminster School, who by tradition are the first to acclaim the sovereign at the ceremony. This was last performed when Queen Elizabeth II was crowned in 1953; she was greeted with "Vivat, Regina! / Vivat, Regina Elizabetha! / Vivat! Vivat! Vivat!", which was incorporated into Hubert Parry's anthem, I was glad.