Hello!
In geometry there are three dimensions.
Something that is one dimensional is neither flat, wide, or tall. It is just a line.
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Something two dimensional is flat and wide but has no height. An example of this is a square.
Something three dimensional is wide and tall, and has flat faces. An example is a cube.
To answer your question something one dimensional is something that is not flat, wide, or tall, such as a line.
Hope this helped!