<?php $tag = get_post_meta($post->ID, 'wikipedia', true); ?>
<?php if (isset($tag) && $tag != '') : ?>
<a href="https://wordpress.stackexchange.com/questions/36825/wikipedia.org/wiki/<?php echo $tag; ?>" title="Wiki this!">Wikipedia</a>
<?php endif; ?>
To explain it a bit: First you put the data in a variable, then if the variable has some data, it will show the link and echo the data after /wiki/. You can use the variable also on other places like so: <a href="https://wordpress.stackexchange.com/questions/36825/wikipedia.org/wiki/<?php echo $tag; ?>">Wikipedia (<?php echo $tag; ?>)</a>