French playing cards are marked '1' instead of 'A'. A cyclops is a creature with one eye and a dromedary is a camel with only one hump. There is only one of lots of things. There is only one President of the United States, there is only one Atlantic Ocean and there is only one you. All of these are unique.
Words beginning with uni- often mean there is one of something.
For example, unicycles have one wheel and unicorns have one horn. Unisex means the two sexes appearing as one because they are indistinguishable by hair or clothing. Mono- can also mean that there is one of something. A monocle is an eyeglass with only one lens, and a monorail is a railway where the track consists of a single rail. The letters A, B, C, D, E, M, T, U, V, W and Y all have one line of symmetry.