Sure, you’re looking for the shortcode API. That would enable you to write a short piece of code that does exactly what you want. As easy as this in your functions.php:
function wpse246274_wiki( $atts ) {
$link = "http://en.wikipedia.org/wiki/" . $atts['link'];
return $link;
}
add_shortcode( 'wikilink', 'wpse246274_wiki' );
Which you would use like this in your post:
<a href="https://wordpress.stackexchange.com/questions/246274/[wikilink link="geography']">Geography</a>
You could even leave out the attribute and just use
<a href="https://wordpress.stackexchange.com/questions/246274/[wikilink]geography">Geography</a>
But that doesn’t look as elegant.