Naming this word type

I studied grammar in some detail at school, but am having real difficulty remembering some of the terminology now. I hope someone can help me find the term I'm looking for.
There is a certain category of words in the English language that will never expand. Whereas new verbs and nouns are being created all the time as things are invented, this particular group is locked. It contains words such as "the," "then," "but," "every," and so on - words that will never be joined by new alternatives. One could draw up a list of these words and consider it complete forever.
Can anyone put me out of my misery and remind me of the name of this category? It's ultimately the list that I seek, but without knowing what they are called collectively, it's impossible to search!