How to make taxonomy terms link to an external site?
Solving this issue for me required three modules:

Link module: Creates a link field for external links. Module page
Lexicon: A module that look for keywords and allow you to set up a lexicon that automatically link keywords to a page explaining them. Module page.
Field Redirection: A module that allow you to create pages that automatically redirect the user to a different page. Module page.


Install the modules and activate them.
Go to Manage fields of the vocabulary you wish to use for external links (structure-->taxonomy-->[your vocabulary]--> Manage fields). Add a link field to the vocabulary and set it's format to "redirect" (this is described in the documentation of the Field Redirection module).
Go to Manage display and hide all other fields (also according to FR module instructions).
Now it is time to set up the Vocabulary. Go to configuration-->system-->lexicon settings. Under the General tab, check "Link terms to the taxonomy term page" and uncheck the options that will make several terms appear on the same page (field redirection will otherwise always redirect to the link associated with the first term on the page).
Link targets are edited for each term in the taxonomy and you add the URL in the "Link" field of each term.


Lexicon is the module you interact with to make the links appear as you want them and any issues will probably be caused by improper setup of the Lexicon. Links/Redirect Field are a small add on that transport users to external websites instead of the lexicon page created by the Lexicon module.