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Lean is the English term—popularized by a team of researchers working under James Womack's direction from the Massachusetts Institute of Technology (MIT)—to describe the system known as the Toyota Way inside the company that created it. For practical purposes, we will consider Lean as a catchword that describes a system approach to producing more with less by empowering employees and by minimizing waste.

To illustrate what waste is, let's review the seven types of waste that Shigeo Shingo identified (along with co-creator Taiichi Ohno, the acknowledged father of Kanban):

Transportation
The unneeded movement of products and/or materials between process steps is waste. Moving things between plants and floors can lead to damage and, most important, create no value for the customer.
In software development, this could equate to the hand-off of some paper documentation from one team to the next.

Waiting
Any time a worker is idle and waiting for something to do, he or she is wasting time.
In software development, this happens when some members of the development team are free but do not have anything they can work on.

Overproduction
Producing more than what the market demands is waste and can lead to other waste, such as high material costs and expensive human resources.
In software development, this is what happens when the team produces features that are not really needed by the users, a phenomenon known as silver plating.

Defects
In software development, this is any deliverable that has errors and/or requires rework.

Inventory
Whenever inventory happens to exist along the assembly line, that is waste. Even if it may be needed, it is still non-value added.
In software development, this is what happens when there are artifacts throughout the life cycle that no one needs to use to create their production.

Motion
Any movement of employees that does not add anything to the process is waste.
In software development, this is any process step that requires an exchange between two team members that does not add anything to the overall process of value creation.

Extra processing
Any processing step that does not add any value to any artifact or product is waste.
In software development, this is what happens when team members have to do certain steps or things that do not directly contribute to the creation of value for the end users.

In addition to these seven types of wastes, an eighth form of waste has been added, which is the "underutilization of human resources."

Even though Lean has been mentioned for quite some time in software development, as witnessed by the writings of some early Lean authors, there were no such Lean processes as Scrum or XP within the Agile space. All of this changed with David Anderson's first known attempt to apply Kanban to software development.

Similarities between Agile/Scrum and Kanban