Variable
An element, feature, or factor that is liable to vary or change. 有able后缀嘛，自然是强调“能变”，所以翻译成变量，即能变的量。
“There are too many variables involved to make any meaningful prediction.”

Variant
A form or version of something that differs in some respect from other forms of the same thing or from a standard.
指的是某种东西的另一种形式，中文应翻译成变体、变种。
“A new variant of the disease has appeared.”