Ubiquity

I don't know what is wrong with my brain, but it doesn't seem to be functioning properly. Last night DH was sitting at the dining room table (after dinner) and looking at the newspaper. He asked "What does 'ubiquity' mean?"

I knew intuitively what the word meant. I understood it's meaning, and could have used it in a sentence, but for the life of me I was unable to state the definition. Fortunately, I was on-line at the time and was able to look up the word and provide DH with the definition. But it bothered me. Not that I had to look up the word for the definition, but that I was unable to fire my brain cells to provide the definition.

I seem to do that a lot. I read words in a book, and I know what they mean without having to look them up. Yet if someone asks me to define a word, even if they provide the sentence for the word so that I understand the context, I find that I am at a loss for words.

I'm not talking about difficult words, I have problems with everyday words too. I think that my brain is going to mush.