Hi,
I would like to create definitions with the command \theorem or in another way, such that
\begin{theorem}[Speed]
This is a definition
\end{theorem}
looks like this:
Peed This is a definition
How can I do this?
General ⇒ creating definition using \theorem
NEW: TikZ book now 40% off at Amazon.com for a short time.

creating definition using \theorem
If you really want to be able to change the name using the environment's optional argument, then you could e.g. copy the definition of the
amsthm package into your preamble and adapt it.
Remember to add the required commands for the at-signs.
proof
environment in the amsthm.sty
file of the 
Remember to add the required commands for the at-signs.