A meaning is 'what it is'. A definition is 'the words we use to describe it'. Something can have a meaning without a solid formal definition (love), and likewise something can have a definition but no useful meaning (see InterCal for some good ones). On this page, we assume the thing has some worthwhile meaning.

The problem is when someone puts more value in the definition than the meaning. Here is a trivial example to prove a point:

Early in history, water was a liquid that dissolved things. Nitric acid was also a liquid that dissolved things. People classified nitric acid as water. Ice was something different than water, although you could get water from ice, and steam was also something different, although you could get steam from water. The definitions at this time were ''wrong'', given the knowledge we have now.

Some people think that a definition is what we make it. If we define water to be a liquid that dissolves things, then that's what 'water' is. Therefore, definitions can never be wrong because they are what we define them to be, by definition. This is also ''wrong''.

A definition ''is only good to the extent that it matches the meaning''. Water is a chemical. Now that we know something about molecular chemistry, water is H2O. That's it. Nitric acid is water mixed with HNO3. Ice is water that happens to be solid, and steam is water that happens to be gaseous. The ''meaning'' of water is much closer to the definition 'H2O' than it is to 'a liquid that dissolves things'.

Sometimes, the meaning isn't clear, as was the case with water in the past. This can cause us to think that our current definition is very good. It can also cause us to think that new definitions are ''wrong'' because we don't understand the meaning those definitions are trying to capture. When the meaning is unclear or under dispute, it is actually useless to rely on definitions. Claiming a definition is wrong in these situations is pointless, since ''the definition is not as important as the meaning''. When better understanding comes about (such as the advent of molecular chemistry in the case of water), it is far more important to strive to understand the new meaning than it is to try to come up with better definitions. Only after the meaning is once again stable and understood is it the right time to focus on definitions again.

That is, unless you write dictionaries for a living. The rest of us are more interested in learning.

Even more contentious is when the same word is used by people to ''mean'' different things. For example, 'the world' can mean variously: 'all the people on this planet', 'this planet', and even 'the entire universe'. This is a matter of language usage, and it is pointless to argue about the ''validity'' of using a particular meaning for a word. Words mean different things to different people. Even 'this planet' means something different depending on who you talk to, since ''people have different understandings of the meaning for the same word or definition''. Again, in these situations it is far more important to focus on the ''meaning'' than the ''definition'' or ''usage'' of a word or phrase.

To illustrate the difference between meaning and definition, consider the difference between understanding and rote memorization.
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See also http://www.astrobio.net/news/article428.html
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Before you accuse someone of being a LanguageAbuser, consider whether you are just getting hung up on definitions. Don't assume IntellectualDishonesty. Otherwise, you'll end up looking like a SelfRighteousJerk.