A Keogh plan is a qualified plan for self-employed individuals. The term Keogh is not a tax term, and you won't find any reference to it in the Tax Code. It's just a bit of retirement planning jargon that refers to the special restrictions placed on qualified plans when they are established by self-employed individuals.
The most onerous restriction is the following: Contributions to retirement plans are often determined by taking a percentage of compensation, but compensation for self-employed individuals is defined differently than it is for employees of corporations. The revised definition often produces a lower contribution limit for a Keogh.