I think we need to first define what we mean by "fifth dimension". 

We typically talk about us living in a 4-dimensional universe. That is, 3 spatial dimensions, and 1 time dimension. Putting that into equation form:
3s+1t=4d$3s+1t=4d$

Viktor Toth talks about Kaluza-Klein theory which is looking at 4s+1t=5d$4s+1t=5d$, making the "fifth dimension" a 4th spatial dimension.

Sherif Kandeel however, looks at the world as 3s+2t=5d$3s+2t=5d$, making the "fifth dimension" a 2nd time dimension.

There are folks who have talked about higher dimension spaces, for example the 10th dimension, with Rob Bryanton (Imagining the Tenth Dimension) really considering 3s+3t+4i=10d$3s+3t+4i=10d$, where he gets to 6 dimensions by treating time as being 3 dimensional like space, and then the sum total being an infinity. Those infinities then behave similarly, getting to 10 dimensions. More classically, M-theory looks at 11 dimensions, which look like 10s+1t=11d$10s+1t=11d$, and String Theory at 9s+1t=10d$9s+1t=10d$

So, I'm going to go with @Viktor Toth, and say that the 5th dimension is a 4th dimension of space (4s+1t=5d$4s+1t=5d$). What IS it? It could be like Carl Sagan describes:

Or it could be something "compactified", as some discussions tackle: Is there any intuitive interpretation of compactification?

Either way, we're stuck at 3s+1t=4d$3s+1t=4d$ for now.