A speck is a very small spot, mark, or piece of something — small enough that you might not see it unless you look. A speck of dust, a speck of dirt on a white shirt, a speck of food on someone's face. It works two ways that feel a bit different. Sometimes it is a MARK on a surface: 'There's a speck on your glasses.' Sometimes it is a tiny PIECE of a substance: 'a speck of dust', 'a speck of paint'. Both are normal English. The word carries a feeling of 'so small it is almost nothing' — that is really the point of choosing it. So people also use it for things that are far away and therefore look tiny: 'The plane was just a speck in the sky.' And they stretch it to things that are not physical at all, usually in a negative sentence: 'There isn't a speck of truth in what he said.' That last use means 'not even the smallest amount', and it is common in speech. One thing worth noticing: a speck is small but it is still one separate thing you could point at. That is why we say 'a speck of dust' but not 'a speck of water' — water does not come in little pointable bits. For liquids people say 'a drop'. The word is everyday, not formal; a child and a professor would both use it.