Short for Supplemental Security Income. It is money the US government pays every month to people who have very little money of their own — people over 65, and people of any age who are blind or have a disability. You say it letter by letter: S-S-I. Two things make it different from a normal pension. First, you do not have to have worked for it; it is paid because you are poor and old, blind, or disabled, not because you paid in over the years. Second, the amount is small — it is meant to cover food, clothes and a place to live, nothing more. Americans often talk about it as if it were a thing you can be on: "He's on SSI." That usually says something about the person's whole situation — no savings, no family money — so people can say it gently or unkindly depending on their tone. It is easy to mix up with Social Security (the retirement money you earn by working) and with SSDI (disability money you also earn by working). SSI is the one that does not need a work history. Outside the US the word means nothing, so if you use it in a conversation with people from other countries, explain it.