Just the everyday definition that people use in normal conversation, not subtle insinuation about political philosophy or anything like that. I’m accountable to my employer for showing up and doing solid work. A student is accountable to their instructor. I’m accountable to the government for my income taxes (and many other laws). It just means that there is a fairly clear set of expectations and a pretty reliable and straightforward process that disincentivizes violation of those expectations in practice (not just in theory).

Why does needing to be reelected not make a politician accountable?