This is the title of the individual responsible for officially recording all events on the field. This person records all the action at the plate, the advancement of runners around the bases, as well as errors, wild pitches, and unearned runs. The last three items on the list are considered judgment calls. The official scorer goes through training to understand how best to make these judgment calls. This record is considered the official record of the game and is used to compile the box score statistics most people are familiar with.
The official scorer is independent and hired by the league, not by the team. Prior to 1980, this was not the case and teams would hire the official scorer. This led to many claims of cheating and inflated numbers. Prior to teams hiring the score keepers, it was left to the responsibility of sports writers. When baseball started to become more popular, teams would try to choose favorable writers to cover stories in order to get favorable statistics printed in the paper. However, all that has now stopped and there is an official, possibly more, at every game to ensure that sanctity of the statistics gathered.