Kaizen (改善, Japanese for "change for the better" or "improvement") is an approach to productivity improvement originating in applications of the work of American experts such as Frederick Winslow Taylor, Frank Bunker Gilbreth, Walter Shewhart,and of the War Department's Training Within Industry program by post-WWII Japanese manufacturers. The development of Kaizen went hand-in-hand with that of Quality control circles, but it was not limited to quality assurance.
The goals of kaizen include the elimination of waste (defined as "activities that add cost but do not add value"), just-in-time delivery, production load leveling of amount and types, standardized work, paced moving lines, right-sized equipment, and others. A closer definition of the Japanese usage of Kaizen is "to take it apart and put back together in a better way." What is taken apart is usually a process, system, product, or service.
Kaizen is a daily activity whose purpose goes beyond improvement. It is also a process that when done correctly humanizes the workplace, eliminates hard work (both mental and physical), teaches people how to do rapid experiments using the scientific method, and how to learn to see and eliminate waste in business processes.
"Kaizen" is the correct usage. "Kaizen event" or "kaizen blitz" are incorrect usage.
Kaizen is often misunderstood and applied incorrectly, resulting in bad outcomes including, for example, layoffs. This is called "kaiaku" - literally, "change for the worse." Layoffs are not the intent of kaizen. Instead, kaizen must be practiced in tandem with the "Respect for People" principle. Without "Respect for People," there can be no continuous improvement. Instead, the usual result is one-time gains that quickly fade.
Importantly, kaizen must operate with three principles in place: process and results (not results-only); systemic thinking (i.e. big picture, not solely the narrow view); and non judgmental, non-blaming (because blaming is wasteful).
Everyone participates in kaizen; people of all levels in an organization, CEO on down, as well as external stakeholders if needed. The format for kaizen can be individual, suggestion system, small group, or large group.
The only way to truly understand the intent, meaning, and power of kaizen is through direct participation - many, many times.
Lean accounting and just in time producton are related concepts.