An installment plan where no interest is applied to the purchase price. The merchant funds it, not the bank — which is why you see it at big chains (Liverpool, Walmart, Amazon MX) and almost never at small businesses. If you pay only the card minimum, the MSI portion still accrues no interest, but any other revolving balance on the card does.
Interest-free months are «interest-free» only if you meet every installment on time. One late payment reactivates interest at card rate — 50% to 70% CAT — and in several contracts it's retroactive to the full original amount. There the «interest-free» becomes the most expensive purchase of your year.
The honest question before accepting an MSI plan: am I buying this because I needed it, or because 12 months make it «look cheap»? If it's the second, don't sign. And watch stacking several plans: each one reserves part of your credit line, and the sum of installments can choke you even if each purchase looks small.