noun
great difference; big difference
Emphasizes a large gap or contrast between two things, often used in comparisons.
彼と私の考え方は大違いだ。
His way of thinking is very different from mine.
去年と今年では景気が大違いだ。
The economy this year is a far cry from last year.
noun
major mistake; big mistake
Used when a mistake is serious or has large consequences; less common than the 'great difference' sense.
計算を間違えたら大違いだ。
If you miscalculate, it'll be a big mistake.
Compound of 大 (おお, 'big') + 違い (ちがい, 'difference'). The 'mistake' sense likely developed from the idea of a large deviation from what is correct.