Voter turnout can be measured in different ways, with different denominators. It can be expressed as a percentage of the number of eligible voters (“voting eligible population turnout”), registered voters (“registered voter turnout”) or the population that is old enough to vote ("Voting age population turnout").
It is quite easy to confuse these different measures of voter participation and make misleading inferences about the relative health of our democracy. This is especially true when comparing turnout in the United States (which is often measured in terms of the voting eligible population or the voting age population) to other countries (which tend to measure voter turnout in terms of registered voters).
For a full discussion of these issues, read Haley Smith's blog, "Voter Turnout: Behind the Numbers."