USA
In the USA the position held by the chief attendant of the bride, typically her closest friend or Sister. If married the title of Matron of Honor is used.
UK
In the United Kingdom, the term "maid of honour" originally referred to the female attendant of a queen. The term bridesmaid is normally used for a bridal attendant. However, when the attendant is married, or is a mature woman, the term "matron of honour" is more commonly used. The influence of American English has led to the Chief Bridesmaid sometimes being called the Maid of Honour.
In the UK, a "maid of honour" is also a type of small cake.