Operational definition of "Length"
You probably think you know what "length" is. But if you try to define it without using the word long, you get tongue tied pretty quickly. We use words such as weight, speed, or length all the time in ordinary conversation without realizing that each of these terms has an "operational definition", which is a description of a property in measurable terms. For example, the operational definition of the property we casually refer to as length is "the change in the location of light over time". That's right, when we say a bookshelf is "a meter long", we are actually saying how long it takes a particle of light to travel from one end of the shelf to the other. (In case you are interested, the answer is 1/299,792,458th of a second. In case you are not interested, that's still the answer.)