Sometimes it helps to hear the same thing different ways, so if I may (and I'm sure Gadfly will correct me if I say something different than what he means by ODT).
The T (Terms) in ODT just means that we are talking about certain sets of words. An Opertational Defined Term is a term (a word or phrase) that has a particular meaning in a given context. For instance. imagine the word "read". All on its own, you don't even know if it is to be pronounced <red> or <reed>. You determine that from the context. Also it can be an adjective, a verb or a noun. As a verb it can be a transitive verb or an intransitive verb. There are at least 8 different meanings it has as a transitive verb from doing what you are doing right now (i.e. to receive or take in the sense of symbols (such as letters) by either sight or touch (think braille) when you read this post. Another meaning is as a synonym for "to indicate", as in the therometer "reads" freezing. We also speak of a computer "reading" a floppy disk. Each of these (and all the other) meanings that might be used of the single word "read" create a new and specific term that has a meaning specific to that context.
So, to speak of an operational defined term is to speak of a term (a word with a specific meaning) in which the meaning is derived from (i.e. defined by) the context in which it is used.
As Gadfly indicated the concept is important when talking about God for the statement "God is love" needs to be defined by that which is actually being discussed at the time, not some other connotations that the reader brings to the context.
Some people think of love as an emotion. But in the context in which it is used in the Bible to say God is love is not to speak of feelings but actions. So, if you will, term "love as used in scripture is defined not by our human experience but by the way God operates. Hence it is an operationally defined term.
And don't think that this is all that difficult. Without knowing it you've used operationally defined terms all your life. For instance, what do you think of when you encounter the word "bait"? If you are one type of fisherman you might be thinking of worms; a different type of fisherman might be thinking of baitfish that are caught and then cut up to use for fishing for other fish; and nonfishermen might have an completely different thing in mind when they think of the word "bait". Each different concept is, in its own unique setting, an operationally defined term with one and exactly one precise meaning meant by it in that context. (I.e., if a catfish fisherman asks for some bait, he does NOT want you to hand him a worm.)
Even "God" (not the divine being himself, but the 3-lettered word "God") is an operationally defined term. Just ask those who seek to argue whether or not Islam, Judaism and Christianity all worship the same God or not. But that leads to another thread we don't need to discuss here.