One, unit, unity, single, solo...
An ace is number one in playing cards. 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 planet Earth, 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.
The letters A, B, C, D, E, M, T, U, V, W and Y all have one line of symmetry.
The international dialling code for the USA and Canada is 01.
A Möbius strip has one edge and one surface. It is easy to make by taking a long strip of paper, giving it one twist and joining together the ends. Ask one of your friends to colour one side of the strip red and the other side green. This turns out to be impossible because the strip has only one side.
A Möbius strip. What happens when you try to colour one side red and the other side green?
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. Monochrome means using only one colour, like a black-and-white photograph. Chemical names often include mono-; for example carbon monoxide is a poisonous gas whose molecules have only one atom of oxygen.