A subdirectory is a folder that sits inside another folder on a computer. If you have a folder called Photos, and inside it a folder called 2026, then 2026 is a subdirectory of Photos. That is the whole idea — a place inside a place, so that files do not all pile up in one heap. The word belongs to programmers, system administrators and technical writing. Everyone else says 'folder', and honestly most programmers say 'folder' too when they are speaking casually. 'Directory' and 'subdirectory' are the older, more precise words, from the days of typing commands rather than clicking icons, and they still live in that world: file paths, documentation, error messages, terminal work. In a path like /home/anna/photos/2026, each slash marks a step down into a subdirectory. Note also that 'sub' here is only about relationship, not size or importance — a subdirectory can hold far more than the folder above it, and it can have subdirectories of its own, going down as many layers as you like.

子目錄（資料夾裡面的資料夾）；一般人講話多半直接說「資料夾」，寫技術文件才用「子目錄」。