ETYM Latin promotio: cf. French promotion.
1. A message issued in behalf of some product or cause or institution; SYN. publicity, promotional material.
2. Act of raising in rank or position.
3. Encouragement of the progress or growth or acceptance of something; SYN. furtherance, advancement.
Or merchandising selling technique used by a business organization to persuade customers to buy its products in the short term. Below-the-line promotion or merchandising would include special offers such as free gifts, discounts, competitions, better value offers (for example, 500 grams for the price of 400 grams), and trade-ins.
Above-the-line promotion is advertising.