Multiple Intelligence (MI) refers to the idea that there is more than one type of intelligence, that you can be smart, even a genius, in one area even if you aren't so bright in others. The best known model of multiple intelligences was developed by Howard Gardner. Unless it is specified otherwise, a reference to MIs is a reference to Gardner's system. It is often referred to in educational settings, and many people think that all schooling should be based on the idea of multiple intelligences, with all curriculum adapted to be compatible with the various intelligences. According to this model, everybody has all the different types of intelligences, but some people have noticeably more of one type or another.
Originally Gardner listed only seven types of intelligence: Linguistic, Logical-Mathematical, Musical, Intra-personal, Inter-personal, Bodily-Kinesthetic, and Spatial. Now he has added at least one "and a half" more -- Naturalistic, and Moral. (Moral is the 'half'). He is always looking for more types of intelligence, so there may be more that I haven't yet heard of.
Linguistic intelligence: the capacity to use language, especially ones' native language, but also other languages. This includes the ability to express what is on your mind through language, and the ability to understand other people. Poets, authors, songwriters, and others who work with language specialize in linguistic intelligence. It is an important intelligence for any kind of writer, orator, speaker, lawyer, or noder.
Logical-mathematical intelligence: the ability to understand the underlying principles of a logical or causal system, the way a scientist or a logician does. It includes mathematics, programming, and other systems governed by formal rules.
Spatial intelligence: this refers to the ability to visualize the spatial world in your mind. Navigation, the ability to read maps, sculpting, and even the ability to play games such as chess.
Bodily kinesthetic intelligence: the capacity to use your body effectively. This might include your whole body, or specific parts of your body. The primary examples of this would be athletes and artists who work with their body and hands, such as dancers, musicians, and sculptors.
Musical intelligence: the capacity to think in music, and to hear, recognize, remember, and manipulate patterns in music.
Interpersonal intelligence: the understanding of other people -- how they think, feel, see you and themselves, and how they will react to a given situation. It is an intelligence that is important for teachers, politicians, salesmen, and even secretaries.
Intrapersonal intelligence: having an understanding of yourself. Knowing what you want, what your abilities are, and what your faults are. People who have strong intrapersonal intelligences tend to be sure of themselves, and since they are less likely to get themselves into situations they can't handle, they are likely to make fewer errors, and seem more competent in their lives.
Naturalist intelligence: refers to the ability to discriminate among living things (plants and animals), and naturally occurring states of nature, such as weather and landscape. Important for hunters, garners, naturalists, farmers, and anyone who enjoys the outdoors. Gardner also speculates that this intelligence may have adapted to the modern world to help us discriminate in the areas of fashion and material goods -- such as recognizing the subtle differences between different makes and models of cars.
Gardner's Definition of Intelligence
According to Howard Gardner (as presented in his book Frames of Mind: The Theory of Multiple Intelligences), human intelligence has the following characteristics:
"A set of skills that enable a person to resolve genuine problems encountered in life".
"The Ability to create an effective product or offer a service that is valued in a culture".
And, "The Potential for recognizing or creating problems, thereby establishing the necessity for the new knowledge".
Criteria for Intelligence
So, what is needed for an area of human endeavor to be considered a full-blown intelligence? First of all, it should be able to be isolated, usually by brain damage or psychological oddity. For example, we have mathematical and musical savants and prodigies, and cases when brain damage has cut out one aspect of intelligence but left the others intact.
Second, an intelligence should have an "identifiable core operation or set of operations", which is to say, you should be able to tell what it does, what it consists of, and perhaps even explain why it evolved to be a part of the human psych.
And a "distinctive developmental history", a progression from beginner to expert. Of course, in most cases the very beginning is early childhood, and the development is often automatic -- for example, learning to walk would be an important part of the normal development of bodily kinesthetic intelligence. Paired with this, an intelligence should have a "definable set of expert 'end-state' performances" -- some sort of definable 'genius level'.
While those seem to be the main qualifiers, things that are appropriate for listing as an intelligence should also have an evolutionary history, or at least evolutionary plausibility, support from experimental psychology and psychometric findings, and "susceptibility to encoding in a symbol system".