>
> %define tde_pkg abakus
> Name: trinity-%{tde_pkg}
>
> Package name is "trinity-abakus"
>
> and,
>
> yum install trinity-abakus
>
> would be the correct command to install the abakus package?

Yes, both are correct.
FYI, it is possible to define macros(*) in terms of other macros, something 
like this:

   %define tde_a  a
   %define tde_pkg  %{a}b%{a}kus
   Name: trinity-%tde_pkg

Still giving package name "trinity-abakus"

So, ideally, resolving definitions should be recursive.

The curly braces are only necessary if there's other text immediately 
following (similar to the rules for $name vs ${name} in bash shell scripting).

This is the most compact link I could find for RPM syntax:
https://rpm-packaging-guide.github.io/#what-is-a-spec-file

 _____
(*) In proper RPM terminology, what we humans would think of as a "variable" 
is considered a really simple "macro" definition.  Any substitutable thing is 
called a "macro", even a simple string substitution like the two defined in 
the example.

Douglas