The support team reports that many admins are not familiar with the term CAPTCHA. It often causes confusion. Technically the measures taken in phpBB do not work against spam in general (e.g. human spammers) so a word involving spam would be misleading. The self explanatory term anti-bot (plugin/measure/...) explains well enough what is meant and is not incorrect either. Visual confirmation should be avoided because not all measures taken against bots are of a visual kind.