Composite Key / Concatenated Key / Aggregate Key:

Composite Key is a primary key, combination of more than one column to maintain unique record. It is also known as Concatenated Key or Aggregate Key

For example:

CREATE TABLE InvoiceDetails (

      InvoiceNo numeric(18,0),

      itemNumber numeric(18,0),

      productId  int,

      quantity   numeric(18,2),

      PRIMARY KEY (InvoiceNo, itemNumber))

InvoiceDetails table may have a composite key on InvoiceNo & itemNumber