Bottom-up estimating is an extremely helpful technique in project management as it allows for the ability to get a more refined estimate of a particular component of work. In bottom-up estimating, each task is broken down into smaller components. Then, individual estimates are developed to determine what specifically is needed to meet the requirements of each of these smaller components of the work. The estimates for the smaller individual components are then aggregated to develop a larger estimate for the entire task as a whole. In doing this, the estimate for the task as a whole is typically far more accurate, as it allows for careful consideration of each of the smaller parts of the task and then combining these carefully considered estimates rather than merely making one large estimate which typically will not as thoroughly consider all of the individual components of a task. In general, the smaller the scope, the greater the accuracy.
This term is defined in the 3rd and the 4th edition of the PMBOK.