阿，或称为阿托 (atto-) 是一个国际单位制词头，符号a，代表10倍。

例如： 1 as = 10 s

Atto can refer to:

• atto-, the SI prefix for a number to the 10 power (for example, 1 × 10 metre is equal to 1 attometre)
• ATTO dyes, a family of fluorescent dye molecules