Tweets are publicly visible by default, but senders can restrict message delivery to only their followers. Users can tweet via the Twitter website, compatible external applications (such as for smartphones), or by Short Message Service (SMS) available in certain countries.[145] Users may subscribe to other users' tweets—this is known as "following" and subscribers are known as "followers"[146] or "tweeps", a portmanteau of Twitter and peeps.[147] Individual tweets can be forwarded by other users to their own feed, a process known as a "retweet".