A computer word. Software that sits between two other pieces of software and lets them work together. Think of a translator standing between two people who speak different languages: neither has to learn the other's language, because the person in the middle passes things across. Common examples: software that lets a shop's website talk to the bank's payment system, or that lets a new app read data out of a very old company database. It is normally an uncountable noun — 'we bought some middleware', 'middleware is expensive' — not 'a middleware', though programmers do sometimes say 'a middleware layer' or, in web programming, 'a middleware' for one small piece of code that every request passes through on its way in (checking who you are, writing a log, then handing it on). That narrower programming use is now very common in frameworks like Express and Django. The word belongs to work and study, not to daily conversation; ordinary computer users never meet it, but anyone reading about how systems are built will see it constantly. It is one word, no space.