A 'Flat' Number?

Not in AmE. We might call those "round" numbers.

I don't understand 'flat' used in this way.

Do you mean a whole number? (no decimal places)

Or an even number? (2,4,6,8,10 etc)

Or a round number? (hard concept to describe...it means no fiddly little digits involved - so we might round 43 to 40 for example, or 1896 to 1800. To go to the nearest number ending in 0).

I think in reality, there isn't a single commonly used term to describe these numbers..

In additional to just saying something is a round number, we can use 'rounded to the nearest ...' to give a little more detail. For example, the number 3,031 could be 'rounded to the nearest thousand', to give 3,000. You could also refer to this as an 'approximation'.

Alternatively, you could refer to the numbers 10, 20, 30, 40.... etc. as "multiples of ten", 100, 200, 300 as 'multiples of one hundred" and so on, regardless of whether they have been rounded or not.

Thank you all, I found your answers very helpful.

Yes, I meant "round numbers." "Flat numbers" are never used in that meaning!

what is a flat number anyway? if you dont mind me asking

Anonymouswhat is a flat number anyway?

So far, nobody's given it a meaning, so flat numbers don't exist. But here's your chance to invent them and become famous.
CJ

the word float normally use in programing language. which represent the decimal numbers. such as 100.12 , 1234.87 ,100.0 and so.
the the word integer use in programing language. which represent the round numbers. such as 100,567,789

It's used in the expression "A flat zero", so only when talking about the number zero as an emphasis.