anything, as deceptive words or actions, used to cover up or gloss over faults, errors, or wrongdoings, or absolve a wrongdoer from blame. 

from dictionary.com

In Toastmasters, a “whitewash” usually refers to an evaluation that lacks critique.  No areas for improvement are suggested.  A whitewash can be quite frustrating as an opportuity to grow is lost.

Often a whitewash will come from an inexperienced Toastmaster, giving an evaluation to an experienced Toastmaster.  We need to remember that there is no such thing as a perfect speech and that every point of view is valid.

To prevent giving a whitewash, incorporate the phrase “I would have liked to have seen you…” into your evaluation.  Hopefully something will follow that you can give to the speaker.

If all else fails and you have no constructive feedback to give, tell the audience up front that you struggled finding an area for improvement and that you will open it up to the audience for suggestions.  Then, ask for suggestions in the middle of your evaluation (making sure to frame, or sandwich, constructive criticism).  This way the speaker will get something to work on and you may learn what to say next time.