I know I've encountered this word before. Examples include almost every editorial and op-ed article.
It's not necessarily written, and it could be a speech.
The article doesn't necessarily persuade the audience, but the writer intends to persuade. It's more respectable and compelling than a screed. It doesn't have the pejorative shadings of propaganda, nor the semi-seriousness of public relations, nor the lofty pretension sometimes associated with eloquence and rhetoric, nor the aggressiveness of argument, nor the passionlessness of opinion.