Multilingual vs. plurilingual
The definitions and the use of the terms multilingual and plurilingual vary considerably. For example on the web you can often find the term plurilingual for things which other people would call multilingual.
Within the context of my Ph.D. work I roughly use these terms in the following way:
  Multilingual refers to the mere fact that materials are available in different languages, like for example a web site which has an English version, a French version, a German version, etc.
  Plurilingual means that the words in the involved languages are more tightly linked to each other.
To give you a concrete example in relation with the dictionary tools:
The multilingual dictionary tool is based on multilingual lexicons which contain translations between several languages. However, there is no further information encoded in these lexicons about how these translations are related to each other.
The plurilingual dictionary tool provides only translations for words which have a strong link between them:
  The Pan-Romance vocabulary comprises those words which are derived from the same Latin word and still have roughly the same meaning (only in few cases two words really have absolute the same meaning!), for example French eau, Italian acqua and Spanish agua come from the Latin word aqua.
  Profile words are those words which are specific in one of the languages and cannot be easily deduced from their graphical form by the learner when comparing it to other Romance languages or the international vocabulary of these languages. Therefore they represent an obstacle for the learner and special emphasis should be put on the proper recognition of these forms to help learners with the comprehension of a text.
 

Graphically similar words with the same meaning are determined dynamically with the help of multilingual lexicons and sophisticated language processing methods.
So called sound correspondences are taken into account to detect all the words which once evolved from the same Latin form but have substituted some parts of the original word with language-specific forms.
For example the word for university in the Romance languages (université - università - universidad) is derived from the Latin word universitas, but now has the endings -té, -dad and -tà. There are many words in these languages with these or similar endings: faculté - facoltà - facultad, possibilité - possibilità - posibilidad, realité - realtà - realidad, etc.