Which channels will you use to acquire your users organically/naturally (without active effort)?
Organic visitors are visitors that are arriving at your website without your active effort and usually without cost. We're talking about users who found your website on Google, on blogs, referrals, etc.
Usually this channel is built overtime, but will last a lot longer than outbound marketing, since this channel is passive and is persistent.
Example: SEO, social media (Facebook, Twitter, Quora, Tumblr, etc.), blogs, content marketing (your blog)