An operational definition is one that defines a term by how you measure it. For example, defining “height” as "the number of axe handles that can fit between a person’s head and feet" is an operational definition. Defining “IQ” as "the score a person gets on an IQ test" is an operational definition. Defining “peanut butter” as "what you get when you grind up peanuts" is an operational definition.
Operational definitions often take the form of numerical measurement, especially in science, because everyone agrees on what numbers mean: A “tall” person is anyone over 6’5”. A “good” score on an exam is "a T-score over 71." An “intelligent person" is "anyone who scores 110 and above on a given IQ test." An “immoral” act is any behavior that is disapproved of by 52% or more of the population. A “hot” day is one in which the thermometer records above 88 degrees F. A “human being” is a fertilized ovum 1 nanosecond after conception. A “frog” is any cluster of at least 8 cells with frog DNA. An “oak tree” is any cluster of 1 or more cells with the potential to become a woody plant over 40 ft tall with leaves that turn red in the fall.
Finally, here's an operational definition of migraine headaches from the KC Star.
1. Does light bother you when you have a headache?
2. Do you get nauseated when you have a headache?
3. Has a headache limited your activities for a day or more in the last three months?
If you answer "yes" to two of the above questions, you have operationally defined yourself as having migraine headaches. This kind of criterion whereby you have have a percentage of a list of symptoms to be diagnosed is the type of operational definition often found in the DSM. It is the field's attempt to make diagnosis more "scientific."
The following are not operational definitions: A big person is one who is tall and heavy. A good score on an exam is one that shows you know a lot. Getting home early is coming in a reasonable time. A hot day is one where the temperature is high. A fetus is a human being. An oak tree a tall plant with a big trunk and a lot of leaves. These are not operational definitions and as a result people can interpret them differently.
Often people differ in their definitions, which can make communication confusing and misunderstood. This doesn't happen when you use an operational definition because you are being specific and explicit about how you are measuring what you are talking about. While someone else not agree with your operational definition, at least they know what you mean.
Because of their clarity and specificity, operational definitions are the standard in the science. Because other scientists need to replicate and verify experimental findings before the results can be accepted as fact, they have to know what the terms mean so they can replicate the experiment. Remember, the ability to replicate experimental findings is a requirement of science.
For example, say that someone did an experiment that showed that "eating breakfast" made people "more productive." If you wanted to repeat the experiment, you would have to know what “breakfast” and “more productive” meant. Is breakfast any meal before 11 a.m.? What if a subject worked a night shift and slept from 3 a.m. to 1 p.m.? And what’s "productive"? Coming up with 1 new project a week? Getting your desk cleaned up? What? How could you replicate this finding if you weren’t sure how they measured their variables? Operational definitions take care of all this.
In addition, operational definitions can be used in everyday life to prevent misunderstanding. Say your spouse says they don’t want to buy an “expensive” new TV and you catch hell because you go out and buy this giant 52" TV for $800 (a $2500 value!). "But Darling," you say, "It was a $5,000 TV on sale!" All of this could have been avoided simply by asking your spouse to operationally define "expensive."
Or say you tell your kid to “be home early,” and they come waltzing in at 1 a.m. When you hit the ceiling they say, “But I came home 'early' just like you said.” The next time you will know to operationally define your terms, e.g.: “By ‘early’ I mean 9 p.m. on the same day.”
Notice the use of quantifying in these examples.