Without specifying a definition, that can easily be true. Yet people manage to have meaningful discussions using those terms on a regular basis. Why? Because either their personal usages match each other, or they find a common definition to work from. Paul and I, for instance, use "fight" very differently. But as long as we agree to use a common definition in a given discussion, we can use that word without it being meaningless.This is true, but often the meaning of a word is opportunistic. Words legitimately have formal definitions, informal definitions, and connotations. There are also sometimes regional or national differences in meaning.
Around here, you will find that the meaning of a word is fluid, and even people whom I believe have a very concrete definition in mind will shuck and jive when convenient. Some words and phrases are more susceptible to this than others.
I believe the following words and phrases are functionally meaningless, beyond very, very broad, sweeping generalization. While they may be well defined, they are so often misapplied they typically do nothing but cause arguments about what they mean, effectively derailing whatever thread in which they appear.
Self Defense
Fight / Fighting
Traditional Martial Art
Sparring
Karate
Effective
Street fight
The street
The following terms are what I consider more settled, even if they do sometimes cause confusion:
Mixed Martial Art(s)
Martial Artist
Grappling
Striking
Choke / Strangle
I'm sure I could think of many more, but these were off the top of my head.
There are some words, like "effective", that are disputed not because of the definition of the word, but because you have to decide a measurement/standard to compare against, and that's even more contentious than the definition of "self-defense".