Depending on who you ask, KEI stands for Keyword Effectiveness Indicator or Keyword Efficiency Index
According to a popular tool used for keyword research - Trellion:
"KEI is designed to measure the effectiveness, or the value of a given search term. KEI is a value between 0 and 10.
The value of each keyword is calculated by correlating a number of factors including the popularity of the keyword and the number of pages on which it occurs. The most effective keywords will be those with a high KEI value. Such terms are typically searched for often, but appear on a small number of pages, and are therefore less competitive."
My 2 cents:
Although I use several tools to determine target keyword terms, whether you are referencing KEI, allintitle/allinanchor, difficulty or the competitiveness of any particular key phrase - less is always more! The higher the level of difficulty/KEI, generally speaking, the harder it will be to rank for that specific keyword term. However, don`t ever rely on one number to give you the data you need when selecting your key phrases - especially if you are actually investing into a link building campaign! Remember, it`s not just how MUCH competition you have but WHAT the competition is doing. KEI should help you narrow down a large list of possibilities, but the ultimate decision should be based off of taking a look at who is already ranking for the desired keyword. When looking at the competition, ask yourself if the efforts you are about to put forth can realistically outdo these guys.
If the KEI is low, but the "other guys" have a large amount of indexed backlinks and pages, it may be best to look for similar term that the competition hasn`t already worked so hard on - unless, of course, you have a large amount of money to invest and a lot of patience.
Anyone else have a question?