The term blond comes from the French word "blund," which described a color between gold and light chestnut, and has gained widespread use. Yellow has never caught on as a popular description for hair.
The term should technically be written as "blond" when describing a male with yellow hair and as "blonde" when describing a female. This is one of the few English adjectives to retain its gendered form. The term "brunet" for dark-haired males or "brunette" for dark-haired females is another hair term that uses feminine and masculine forms.
The French word "blund" is thought to originate from the Latin word "blundus," which is an improper pronunciation of the word "flavus," which means yellow. There is some confusion as to whether the word blond emerged from the German word blonden-feax, because the Spanish, Italian and Old Provençal forms all come from the German word. However, the French endings suggest a French origin for the word.
The word blond first appeared in the English language in 1481, and was later reintroduced in the 17th century. Before that time, English speakers used the word "fair" to describe people with blond hair.
Some might argue that the term blond has taken on meaning beyond hair color, suggesting whimsical or spontaneous personality traits.