A spillover is what happens when something that starts in one place does not stay there — it runs out and affects something next to it. Picture a cup that is too full: the water that goes over the edge is the spillover. English uses that picture for situations, not just liquid. If a war in one country makes its neighbour take in many refugees, that is a spillover. If your stress at work follows you home and you are short with your family, that is spillover too.

It is usually a noun, and you often see the pattern 'a spillover from X' or 'the spillover of X into Y': 'the spillover from the banking crisis', 'a spillover of violence into the next town'. It is also used before another noun: 'spillover effect', 'spillover damage'. You will meet it a lot in news, business writing, and economics, where it has a slightly technical feel — economists talk about 'spillover effects' when one country's policy changes another country's economy.

Two things worth noticing. First, the meaning is usually bad or at least unwanted, because problems spread more visibly than good things do — but it is not always bad. People do say 'a positive spillover', for example when a new train station brings business to the whole area. Second, there is an older, plainer use: the extra people or things that do not fit. A church might open a 'spillover room' for the crowd that will not fit in the main hall. That use is still alive and is completely neutral.

British writing sometimes keeps the hyphen, 'spill-over'; American writing almost always writes it as one word. The verb phrase behind it stays two words: the problem 'spilled over into' other areas.