This is the 40th most frequent Japanese word.
一
The Japanese word "一" means "one."
The term "一緒に" (issho ni) is used to mean 'together.'
一緒に映画を見ますか？
Shall we watch a movie together?
Here, "一つ" (hitotsu) is used to indicate 'one' item or unit of something.
一つお願いします。
One, please.
In this sentence, "一番" (ichiban) expresses the concept of being 'number one' or the best in rank or order.