Sp5 hybridization in cyclopropane?
Until this post, $\mathrm{sp^1,~sp^2~\text{and}~sp^3}$ meant for me, that the hybrid orbitals would consist from one s orbital and one, two or three p orbitals. I thought of it being so, as this seems to be what the most textbooks suggest, as can be seen in the following image:



One 2s orbital and three 2p orbitals hybridize to four $\mathrm{sp^3}$ orbitals.

This scheme seems to work but it also seems to be a working simplification as can be seen by my question about $\mathrm{sp^5}$ hybrid orbitals, which simply cannot consist from one s orbital and five p orbitals, as there are only three p orbitals in reach.

Then what does this nomenclature actually mean?

As Martin commented, it is more like the following:
$$\mathrm{sp^n \equiv s^\frac{1}{n+1}p^\frac{n}{n+1}}$$ or to say it slightly different, a $\mathrm{sp^n}$-bond has 
$\frac{100}{n+1}~\%$ s-amount and $\frac{100\,n}{n+1}~\%$ p-amount.

\begin{array}{lll}
\hline
\text{bond} &amp; \text{s-amount} &amp; \text{p-amount}\\
\hline
\mathrm{sp} &amp; 50~\% &amp; 50~\%\\
\mathrm{sp^2} &amp; 33~\% &amp; 67~\%\\
\mathrm{sp^3} &amp; 25~\% &amp; 75~\%\\
\mathrm{sp^4} &amp; 20~\% &amp; 80~\%\\
\mathrm{sp^5} &amp; 17~\% &amp; 83~\%\\
\ldots &amp; \ldots &amp; \ldots \\
\mathrm{sp^n} &amp; \frac{100}{n+1}~\% &amp; \frac{100\;n}{n+1}~\%\\
\hline
\end{array}