The term 'prefix meaning 'OUTER'' can be solved by three different crossword solutions: EXO (3 letters), ECT (3 letters), and ECTO (4 letters). EXO is derived from the Greek word 'exo, ' which means 'OUTSIDE' or 'OUTER.' ECT is an abbreviation of the Latin word 'exterus, ' which also means 'OUTSIDE' or 'OUTER.' Lastly, ECTO is a combination of the two words 'exo' and 'terus, ' which together mean 'OUTER.' All three of these prefixes are used to describe something that is located on the OUTSIDE or exterior of something else.