A loading spinner or a spinning pre-loader is an image (usually animated) in which an object or a set of objects spin around some axis indicating a content loading process. Sometimes it is referred to as a throbber or a round pre-loader. Mostly these animations are placed in a transparent square image. This kind of illustration is considered most optimum as, usually, it doesn't take a lot of interface space, does not usually require a lot of frames (making it lighter in terms of bytes), while clearly shows that some action is being taken on the background. It has become quite a standard and even non-IT professionals understand that they need to wait for a process to complete when the spinner is shown.
The "circular" category mostly consists, but is not limited to spinners. There are some animations (like Growing ring), that form a circle in one or another way during animation. Some animations may be thematic (e.g. House loading, which may be perceived as a construction process), while most of them are created in a general "any-use" form (for example, most popular chasing arrows spinner.
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Circular (Spinners)