Dictionary.com
definitions

safe period

noun
  1. an interval of the menstrual cycle when fertilization is considered to be least likely, usually a number of days prior and subsequent to the onset of menstruation.

Origin of safe period

First recorded in 1915–20
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for safe period

safe period

noun
  1. informal the period during the menstrual cycle when conception is considered least likely to occurSee also rhythm method
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
safe period in Medicine

safe period

[sāf]
n.
  1. The period in the menstrual cycle when conception is least likely, typically from ten days before to ten days after the onset of menstruation.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.