parse content from wikipedia article replacing search term with the title of article

You can’t and shouldn’t echo something into a function’s arguments. The single_post_title() function will echo the title by default. You should either set the second parameter to false, or use another function:

echo getDescription( single_post_title( '', false ) ); 

WordPress also offers the get_the_title() function that returns the single post’s title which you can use here.

So, your code should be:

echo getDescription( get_the_title() );