Use the adjective implicit when you mean that something is understood but not clearly stated. You might think you and your boyfriend might have an implicit understanding that you are going to get married, but it's probably better to talk it through.
A very near synonym of implicit in this particular meaning is the word implied. But the adjective implicit also means "complete without any doubt," so we can say that we have implicit trust or confidence in someone. The Latin root implicāre means "to involve or entangle." Another English word with a more obvious connection to the Latin is the verb implicate.