Which SI prefix denotes 10 to the power of minus 15?

Femto, from the Danish and Norwegian femten, fifteen - one of two prefixes taken from Scandinavian rather than Greek or Latin, alongside atto from atten, eighteen.

The femtosecond is the natural unit of chemistry. A chemical bond vibrates on that timescale, so a femtosecond laser pulse is short enough to photograph a molecule mid-reaction rather than before and after - the field of femtochemistry, which won a Nobel in 1999 and turned reaction mechanisms from inference into observation. For scale, a femtosecond is to a second roughly as a second is to 32 million years.

The femtometre is the natural unit of the nucleus, and is informally called the fermi. An atomic nucleus is a few femtometres across while the atom is about 100,000 times larger, which is why Rutherford's foil experiment was so startling: almost everything went straight through, because almost everything is empty. The prefix system now runs from quetta at 10^30 down to quecto at 10^-30, the last four added in 2022 mainly because data storage was about to run out of names.