Xenno-

The SI prefixes above are not the only extreme ones. Others such as xenno (x) (10⁻²⁷) and xenna (X) (10²⁷), or vendeko (v) (10⁻³³) and vendeka (V) (10³³) exist, and can help simplify the expression of extreme numbers.

'Now we're talking xennobiology!' [...] a complex organism based on similar processes to the primitive ones they'd seen probably would be about a xennometre in size.

From Nanotech to Xennotech [...] A one-kilogram hole has a radius of about 10⁻²⁷ meter.