underestimate

Short definition from Wiktionary

Verb

1. 1
   To estimate too low; to perceive (someone or something) as having a lower value, quantity, worth, etc., than what he/she/it actually has.
2. 2
   To perceive or expect (someone or something) to be less significant or difficult than it actually is.

Noun

1. 1
   An estimate that is too low.