By etymology "survey" (sur- from 'super', as in 'from above' and -vey from 'view') means to get an overview, not the full picture. So long as the 25% was truly random and not i.e. self-selected (opt-in) then it quite meets the definition of the term.

By etymology "survey" (sur- from 'super', as in 'from above' and -vey from 'view') means to get an overview, not the full picture. 

So long as the 25% was truly random and not i.e. self-selected (opt-in) then it quite meets the definition of the term. If the survey is optional, then the answers will be representative only of those who feel a need to answer. For instance, imagine a restaurant in which one could fill out a feedback card after dining. Even if most diners are happy, most of the feedback will be negative because the happy customers see little reason to give feedback.