When you suggest a person for a position in the government, or propose a theme for your school's prom, you nominate that person or idea, especially if it's done in an official way.
In the United States, political parties hold primary elections and conventions to nominate candidates for president. The word nominate originally meant "to call by name," from the Latin word for "name," nomen , but by 1600 it began to be used to talk about politics.