The only Shakespeare play that contains the word “assassination” is Macbeth. Macbeth uses the word while debating whether to murder King Duncan: “If th’ assassination / Could trammel up the consequence...” The term appears during his private struggle between ambition, morality, and fear of punishment.
The word was unusually new in Shakespeare’s period. The Folger Shakespeare Library notes that Shakespeare appears to have invented “assassination” for this play, although related words such as “assassin” already existed. Macbeth does not calmly call the planned act murder; his hesitant, indirect language reflects his reluctance to name what he intends to do.
The play’s plot contains several political killings, including Duncan’s murder, Banquo’s assassination, and the slaughter of Macduff’s family. That makes Julius Caesar a tempting wrong answer, but Shakespeare’s Roman tragedy discusses assassination without using the exact word. Macbeth was probably written around 1606 and was first printed in the 1623 First Folio.