Kaizen is the Japanese word meaning continued, gradual improvement -- as compared with breakthroughs and revolutions; usually used in a business context.
Historically, the term has become important because certain Western commentators use it in the context of saying things like: The Japanese are good at kaizen; that is gradual improvement of a technology, but no good at innovation and breathroughs. For example, the Japanese did not invent video cameras (a breakthrough), but once the grasped the idea, they refined it, made it better, smaller, faster, cheaper (this is kaizen).
For me, this has slightly racist overtones; besides the fact that it's also wrong. The Japanese did come up with the idea for the original Walkman, for instance, and as well as that have been doing kaizen on them for 30 years, and they're still getting smaller, faster, cheaper. It's usually used by older Americans to explain why they're losing so much high-tech industry to Japan. It suits the psyche of many to say the equivalent of "we do all the hard work, and the Japanese enjoy the fruits of our labour".