soapbox

Short definition from Wiktionary

Noun

1. 1
   A crate for packing soap.
2. 2
   A small platform, being or resembling a literal soapbox, on which a person stands in order to give an impromptu speech or espouse his or her views.
3. 3
   A platform or outlet, such as a media platform, that a person uses to express his or her opinions.
4. 4
   A figurative means of talking repeatedly or at length about one's opinions on one's pet topic.
5. 5
   A figurative means of talking repeatedly or at length about one's opinions on one's pet topic: The topic itself.
6. 6
   A soapbox car.

Verb

1. 1
   To give a speech from (or as if from) a soapbox.