ac·ci·den·tal 
/ˌæksəˈdɛntl̟/ adjective
/ˌæksəˈdɛntl̟/ adjective : happening in a way that is not planned or intended : happening by accident
▪ an accidental discovery of oil ▪ The timing of the announcement was purely accidental. : happening as an accident ▪ an accidental shooting ▪ The death was ruled accidental.
▪ an accidental discovery of oil ▪ The timing of the announcement was purely accidental. : happening as an accident ▪ an accidental shooting ▪ The death was ruled accidental.
— ac·ci·den·tal·ly /ˌæksəˈdɛntl̟i/ adverb
▪ He accidentally deleted the file. ▪ The lock prevents the gate from opening accidentally. ▪ Did it happen accidentally [=by accident] or intentionally? ▪ He bumped into her accidentally on purpose. [=he bumped into her on purpose but he tried to make it appear to be an accident]
/ˌæksəˈdɛntl̟i/ adverb ▪ He accidentally deleted the file. ▪ The lock prevents the gate from opening accidentally. ▪ Did it happen accidentally [=by accident] or intentionally? ▪ He bumped into her accidentally on purpose. [=he bumped into her on purpose but he tried to make it appear to be an accident]
