What is the definition of basal body temperature?
Family Planning: A Global Handbook for Providers, a 2012 WHO publication related to calendar-based contraceptive methods:
Basal body temperature – the body temperature of a fertile woman when resting – rises after ovulation and remains high until the next menstruation.
Detailed method: Every day after waking up, immediately (before getting up or eating any food) measure body temperature and record it in a special designed form; after ovulation, body temperature usually rises 0.2-0.5°C/0.36-0.9°F (this usually happens at the middle stage of the menstrual cycle).
Wikipedia:
Basal body temperature is the lowest body temperature attained during rest (usually during sleeping). It is generally measured immediately after awakening and before any physical activity has been undertaken.