Duplicate bridge mislead
The relevant rule is Law 45 (C) from the Laws of Duplicate Bridge (2017)


  LAW 45 - CARD PLAYED
  
  ...
  
  C. Card Deemed to be Played
  
  
  A defender’s card held so that it is possible for his partner to see its face is deemed played
  to the current trick (if the defender has already made a legal play to the current trick, see
  Law 45E).
  Declarer is deemed to have played a card from his hand if it is:
  (a) held face up, touching or nearly touching the table; or
  (b) maintained in such a position as to indicate that it has been played.
  A card in the dummy is played if it has been deliberately touched by declarer except for the
  purpose either of arranging dummy’s cards, or of reaching a card above or below the card
  or cards touched.
  (a) A card is played if a player names or otherwise designates it as the card he proposes to
  play (but see Law 47).
  (b) Declarer may correct an unintended designation of a card from dummy until he next
  plays a card from either his own hand or from dummy. A change of designation may be
  allowed after a slip of the tongue, but not after a loss of concentration or a
  reconsideration of action. If an opponent has, in turn, played a card that was legal before
  the change in designation, that opponent may withdraw the card so played, return it to
  his hand, and substitute another (see Laws 47D and 16C1).
  


So to answer your question: No, a Defender's card is not played simply because an opponent (Declarer) can see which card it is; rather it is played if it is held so that the Defender's partner can identify it,

Further clarification through Minutes of the W.B.F. Laws Committee, where necessary, are available here.