Final answer:
The term 'Spinnbarkeit' refers to the stretchy and pliable nature of cervical mucus during ovulation in natural family planning.
Step-by-step explanation:
In the context of natural family planning, the term Spinnbarkeit refers to the property of cervical mucus becoming stretchy and more pliable during ovulation. This allows the mucus to be easily stretched between the fingers without breaking. The changes in cervical mucus consistency during the menstrual cycle can help track fertility and determine the best time for conception.