Imagine that we didn't speak the same language, and you want to learn what my words mean. I might point to a tree and say "blork." You don't know if I just taught you the word for tree, bark, leaf, trunk, green, wood, plant, tall, solid, or over there. I would have to point to several other trees before you completely understood that I was actually teaching you the word for "maple."
Now let's say that there is a river with fish swimming in it, and the sky is full of birds flying overhead. I point to a sparrow and say "flird." Later, I point to a trout and say "swish." This goes on for some time. A hawk flies over and I say "flird," a salmon swims by and I say "swish," a crane flies over and I say "flird," a bass swims by and I say "swish," and so on.
By now, you think you've figured it out. "Flird" means "bird," and "swish" means "fish." Simple. For the next several days, you seem to be using my words correctly. Then one day, a duck swims by on the river. You proudly point to it and say "flird," because the duck is indeed a bird. I look confused, shake my head, and say that the duck is "swish." At first, you think I'm insane, because I think that the duck is a fish. After a while, it finally hits you that I was teaching you verbs, not nouns. "Flird" means "fly," and "swish" means "swim."
This type of miscommunication as to intent and meaning can get people into semantic arguments which have real world impacts. If one person thinks that "man" means "person in a masculine role" and "woman" means "person in a feminine role," but the person you're speaking to thinks that "man" means "person with a penis" and "woman" means "person with a vagina," then there is going to be confusion.
If one person thinks that "gay" means "person actively having sex with a same-sex person," but the other person thinks that "gay" means "person predisposed to find a same-sex person attractive," then when the debate comes up as to whether gay is a choice or not, they're going to be arguing from two completely different perceptions.
The problem comes up in translations of old languages, such as Biblical texts, as well. People like to point out that the creature from the story of Jonah was not a whale, because the English translation calls it a "fish," and whales are not fish. However, if the translation from ancient Hebrew didn't distinguish between different types of large marine animals, it would be silly to assign modern English classifications. In some cases, the entire meaning of the story can be called into question. We use the word "virgin" to describe someone as-of-yet sexually inactive, but if Aramaic didn't distinguish between sexual inactivity and youth or unmarried status, then the conception by Mary becomes a bit less miraculous.
In politics, this type of inconsistent definition can also cause communication problems. One person might say "libertarian" to mean avoiding government overreach or overregulation, while another might be using the word to mean borderline anarchy. One person might say "liberal" to mean lack of restriction on people's rights, while another might use it to refer to lack of government restraint. One person might say "conservative" to mean "proceed with caution," while another might mean "regress." One person might say "anarchist" to refer specifically to anarcho-communists, while another may be referring broadly to any anti-authoritarianism. One person might say "Christian" to refer to all religious subcategories which include Christ as a central figure, while another may use the same word to refer specifically to congregational Protestant denominations.
My point is that getting hung up on the definitions of words is much deeper and more important than just debating whether the definition of "sandwich" applies to hot dogs, or "planet" to Pluto. Personally, I don't care what the definitions of words actually are, so long as we all agree on what they mean before we get ready to kick someone's ass for using them incorrectly.
