Ratios are most often notated as a:b, rather than a/b, which is said "a to b". For example, if I have ten bananas and four apples then I could say the ratio of bananas to apples is 5 to 2 (or 10 to 4). So in your situation you could say:
Voter turnout is the ratio of votes cast to registered voters.
To emphasize that a is a sub-set of b we often use the phrase "out of" or "in" (these might sound opposite, but they actually mean the same thing), especially when giving the actual number. A very common type of statement would be something like
One out of/in ten* registered voters actually cast a vote in
the recent election. This ratio is a little higher than the previous election, although in general voter turnout has been declining for the past several decades.
This language for ratios of part-to-whole works well for the non-mathematically-inclined who might be thrown off by mathier terms like "divided by" or "fraction" or "quotient". (I know people who have a hard time with the idea of "one divided by three" but no problem at all with "one in three"). Unfortunately, it is a little awkward for a purely explanatory sentence, with no numbers. You could perhaps say:
The voter turnout ratio is the number of voters who actually voted out
of the total number of registered voters.
However, I might stick with the "to" language, just changing your terminology a bit to make the relationship clear:
Voter turnout is the ratio of registered voters who actually cast votes to total registered voters.
*All numbers there are fictional.