An excuse was a reason or explanation to defend or justify a fault or offense. Some excuses were lies, while others were only part of the reason why an action was taken.
In 2370, Admiral Mitsuya diverted the USS Crockett to Deep Space 9, ostensibly to discuss Cardassian foreign policy. In reality, according to Jadzia Dax, the visit was an excuse to play poker with Benjamin Sisko. (DS9: "Paradise")