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What does the jargon filesystem mean?

I know this question sound quite stupid. Sometimes when I go through sections of Linux books about filesystem, it may turn out that it is a chapter talking about the directory structure of a standard Linux system, like /proc, /dev, /etc, /usr directories, with a brief explanation. Sometimes it may be about filesystem types like ext3, ext4 and nfs, dos, etc. I feel quite confused about that. Which one is correct? Or did I miss something?

In short, my question is:

The word filesystem means directory structure of Linux system or things like ext3, ext4, fat32, etc.?