A lie — but the polite, careful word for one. Both 'lie' and 'untruth' mean a false statement, yet they feel completely different in the mouth. 'You lied' is an accusation. 'That was an untruth' keeps a cool distance, and lets a speaker point at the false thing without directly attacking the person. You hear it where people must be careful: in court, in politics, in newspapers, in official apologies. A politician who will not say 'the minister lied' will say 'the minister told an untruth'. It is countable — one untruth, several untruths — and the usual verb is 'tell': tell an untruth. Two more things worth noticing. First, it can be gentler in another direction too: adults sometimes call a small child's story an untruth to avoid the heavy word 'lie'. Second, there is an older, rarer use meaning simply 'the state of not being true' with no accusation in it at all, but you are very unlikely to meet that one outside literature.

謊言、不實之詞（比 lie 委婉、正式的說法，接近中文的「不實陳述」）