Uniforms and ranking
Aikido practitioners, commonly called aikidoka, generally progress by promotion through a series of "grades" (kyū), followed by a series of "degrees" (dan), pursuant to formal testing procedures. Most aikido organisations use only white and black belts to distinguish rank, but some use various belt colors (段位, dan'i?). Testing requirements vary, so a particular rank in one organization is not always comparable or interchangeable with the rank of another.[1]
The uniform worn for practicing aikido (aikidōgi) is similar to the training uniform (keikogi) used in most other modern martial arts; simple trousers and a wraparound jacket, usually white. Both thick ("judo-style"), and thin ("karate-style") cotton tops are used. Most aikido systems also add a pair of wide pleated black or indigo trousers called a hakama. In many styles its use is reserved for practitioners with black belt ranks, while others allow all practitioners or female practitioners to wear a hakama regardless of rank.[1]
Source : http://en.wikipedia.org/wiki/Ki-Aikido