I never claimed that definitions don't exist. But yes, I believe that words are defined based on how people use them, and each person is going to have a slightly different understanding of the meaning of any one word.

You seem to be mixing up meaning and definition. If you have learnt a version of the word "woman" by which an adult female giraffe is a woman then you are not wrong to use the word this way, but this would not change the definition of the word unless there was a significant group of people or contexts in which the word was used this way.

You have claimed woman means whatever any individual says it means

Again, no, I haven't.

If you just claim that the word "woman" means that, that does nothing at all. What you say has no influence whatsoever on the meaning of a word.