What does the "s.d" in "s.d = 21.2" (from a medical research) mean?

Standard Deviation

Thank you Josie.

You are welcome

@josie I had entered in google medical reaearch dictionary, went to:
http://www.medicinenet.com/script/main/hp.asp
and entered:
s.d as well as s.d=21.2 it just keeps removing the dots, and says no result.
Would you let me know how you got there?

Standard deviation is a statistical concept, not a medical one. It comes up in papers besides medical journals, like sociology or economic research.

Thanks @xxii. I thought it would come up anyway, in the medical research dictionary. I got it from the Amagydala story in the news a few weeks ago.