According to www.onelook.com a synonym is two words that can be interchanged in a context are said to be synonymous relative to that context.
In comparing lithium and sodium, I could say "lithium and sodium are similar" or "lithium and sodium are related" and mean the same thing. Therefore I conclude that 'similarity' and 'relationship' are synonyms, at least in this context. Furthermore they are listed as synonyms on www.thesaurus.com. Bensaccount 17:06, 19 Mar 2005 (UTC)
In comparing an apple and an orange, I could say "apples and oranges are similar, in that they are both fruits" or "apples and oranges are related, in that they are both fruits". Bensaccount 17:12, 19 Mar 2005 (UTC)