Definition of "understudy"

understudy

/ˈʌndɚstʌdi/

verb
third-person singular simple present understudies, present participle understudying, simple past and past participle understudied
1. To study or know a role to such an extent as to be able to replace the normal performer when required.
2. To act as an understudy (to someone).
3. To act in a similar manner to some known person.
  Quotations
  That lean gentleman had apparently understudied Pharaoh of old and hardened his heart.
  1935, Owen Conquest, Morny's Master Stroke
noun
plural understudies
1. A performer who understudies; a standby.
  Quotations
  For Wynne-Edwards, the role of the outcasts in the welfare of the group is to wait in the wings as understudies, ready to step into the shoes of any territory holder who dies on the main stage of group reproduction.
  1976, Richard Dawkins, The Selfish Gene, Kindle edition, OUP Oxford, published 2016, pages 153–154