Is host web always site collection

For a SharePoint hosted application, is the host web always referred to a site collection, and it cannot be a web only ?

Let assume I install a sharePoint hosted application at this address - which is a sub-sub web,

site collection - http://ift.tt/1w9llrY

http://ift.tt/1w9lls0

So is site collection host web or sub-sub-web is host web.