Then what's the difference between the two terms?
I've always thought that they were quite different (RMS vs ESR). Especially when capitalized as pronouns.
Edit: Nevermind. I missed ajross's response.
The more relevant one for the current discussion: people who use and identify with the term "Free Software" generally care about software freedom as a valuable principle independent of its practical benefits (in other words, software should be free because it's wrong to restrict developers' ability to use, copy, modify, and distribute it), while people who use and identify with the term "Open Source" generally care most about the practical benefits (software should be open because that's the most effective and efficient way to produce software). There's a great deal of overlap between those two communities, but in general those two terms imply a different set of underlying principles held by the speaker. People who want to be all-inclusive sometimes say "Free and Open Source Software" or FOSS.
The origins of that distinction come from the original creation of the term "Open Source", often described as a marketing campaign for Free Software: the term often helps win over people (and businesses) who are unconvinced by software freedom as a principle but highly convinced by the effectiveness of Open Source as a software development model.
In terms of literal definition, Free Software is defined by the Free Software Foundation's "four freedoms" (https://www.gnu.org/philosophy/free-sw.html): use/run, study/modify, redistribute, and redistribute modified versions. Open Source is defined by the Open Source Initiative's Open Source Definition (http://opensource.org/osd), which in turn was derived from the Debian project's "Debian Free Software Guidelines" (DFSG, http://www.debian.org/social_contract). In practice, almost every license that passes one definition passes the other; it's exceedingly rare to find a license that's actually Open Source but not Free Software or vice versa.
Also worth noting: both terms are unfortunately ambiguous, in that "free software" (especially when not capitalized) can be confused with software available for no fee, and "open source" can be confused with software with source available but not under an Open Source license.
There is a subtle distinction. Open Source is generally in favour of developers' freedom, Free Software is generally in favour of users' freedom. These come into conflict when, for example, developers want to lock their users into SaaS walled gardens. Open Source advocates are generally in favour of the developers' freedom to do this. Free Software advocates are generally in favour of the users' freedom to not be locked in (see the AGPL for example).