当且仅当
dāng​qiě​jǐn​dāng

if and only if

  1. 当 當 dāng prepositionadverb
    at a time
    At a given time or place; just as; while; when (often used in the pattern 当...的时候 dāng... de​shí​hou)
  2. 且 qiě conjunction
    and
    and; also; used to link adjectives or verbs to show a parallel relationship
  3. 仅 僅 jǐn adverb
    only
    only; merely; barely; used to limit a scope or indicate a small quantity
  4. 当 當 dāng prepositionadverb
    at a time
    At a given time or place; just as; while; when (often used in the pattern 当...的时候 dāng... de​shí​hou)