A definition for an English word can be requested by going to Wiktionary:Requested entries (English) and putting it in the appropriate section.
A word in another language can be requested by going to Category:Requested entries or this automatically generated list and finding the right page (or replace “English” by the appropriate language name in the above link). If the page for the language does not exist yet, you can either create it yourself or put up a request for it at Wiktionary:Tea room.
A word in a language you don’t know can be requested by inserting it at the pages for unknown Latin-script, Arabic-script, or CJK-script words, or the pages for other scripts:
If you are looking for a word that captures a meaning that you have in mind, you can consult the Tea Room.