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Period of slow development (9 letter answer)
The answer GESTATION is so rare that it appears less than once every 2500 puzzles.
An elephant has a long one
GESTATION as a noun:
1. (gestation, gestation period) = the period during which an embryo develops (about 266 days in humans)
2. (pregnancy, gestation, maternity) = the state of being pregnant; the period from conception to birth when a woman carries a developing fetus in her uterus
3. the conception and development of an idea or plan