An excerpt from Wikipedia (Feb 26, 2015):
Therbligs are 18 kinds of elemental motions used in the study of motion economy in the workplace. A workplace task is analyzed by recording each of the therblig units for a process, with the results used for optimization of manual labor by eliminating unneeded movements.
The word therblig was the creation of Frank Bunker Gilbreth and Lillian Moller Gilbreth, American industrial psychologists who invented the field of time and motion study. It is a reversal of the name Gilbreth, with 'th' transposed.
A basic motion element is one of a set of fundamental motions required for a worker to perform a manual operation or task. The set consists of 18 elements, each describing a standardized activity.
Transport empty [unloaded] (TE): reaching for an object with an empty hand. (Now called "Reach")
Group (G): grasping an object with the active hand.
Transport loaded (TL):moving an object using a hand motion.
Hold (H): holding an object.
Release load (RL): releasing control of an object.
Preposition (PP): positioning and/or orienting an object for the next operation and relative to an approximation location.
Position (P): positioning and/or orienting an object in the defined location.
Use (U): manipulating a tool in the intended way during the course working.
Assemble (A): joining two parts together.
Disassemble (DA): separating multiple components that were joined.
Search (Sh): attempting to find an object using the eyes and hands.
Select (St): choosing among several objects in a group.
Plan (Pn): deciding on a course of action.
Inspect (I): determining the quality or the characteristics of an object using the eyes and/or other senses.
Unavoidable delay (UD): waiting due to factors beyond the worker's control and included in the work cycle.
Avoidable delay (AD): waiting within the worker's control which causes idleness that is not included in the regular work cycle.
Rest in peace (R): resting to overcome a fatigue, consisting of a pause in the motions of the hands and/or body during the work cycles or between them.
Find (F): A momentary mental reaction at the end of the Search cycle. Seldom used.