Getting less money than your work is really worth. The important thing is that this word carries a judgement, not a measurement. Nobody can look at a payslip alone and see 'underpaid' — you have to have decided that the work deserves more. That is why it is such a common word in complaints, union talks, and newspaper articles about nurses, teachers and care workers: saying someone is underpaid is arguing for them. It describes people (underpaid staff, an underpaid teacher) and it very often turns up in the fixed pair 'overworked and underpaid', which is close to a set phrase in English — half joke, half complaint, and something people say about themselves all the time. You can also use it of the job or the work: an underpaid job, underpaid labour. Grammatically it is the past participle of the verb 'underpay', so it works both as an adjective ('she is underpaid') and as a real passive verb ('she was underpaid for years', 'they underpaid her by two thousand dollars'). Those two are subtly different: 'she is underpaid' means her salary is unfairly low; 'they underpaid her' can also mean a plain accounting mistake — they paid less than the agreed amount. Context tells you which. Its opposite, 'overpaid', is almost always an insult.