I would create custom post types and custom taxonomy, taking the standard “Posts” out of the loop. You can then have two separate Taxonomies (Tag systems) for synonyms and antonyms.
http://codex.wordpress.org/Post_Types
http://codex.wordpress.org/Taxonomies
You can then use a plugin like Gravity Forms to have your users register for the site and submit their words, which can be turned into posts automatically.
If this is all greek you may want a WordPress dev for help. Let me know if you would need help and if this answer helps!