The term "guinea" has several meanings depending on its context.
1) In a historical context, it refers to a coin that was first issued in England in 1690 and was made from gold. The name came from the fact that the coin was worth 21 shillings, which was equivalent to one pound, 1 shilling and 3 pence - hence "a guinea for a guinea".
2) In North America, particularly in Canada and the United States, it is used as a term for a large-denomination banknote, usually $50 or $100. This usage comes from the fact that early paper money issued by banks were often printed on one side only and had guineas depicted on them to indicate their value.
3) In South Africa, "guinea" is used as a term for a type of small coin worth 50 cents or half a rand. This usage comes from the fact that these coins replaced the earlier guinea-sized coins issued by the South African government in the early 20th century.
4) In some parts of Africa, particularly West Africa, "guinea" is used as a term for a type of large bird with distinctive plumage and loud calls - the guineafowl. This usage comes from the fact that these birds were introduced to many parts of the world by European settlers who brought them along as a food source.