derailment
n. C / U C1 Advanced US //dɪˈɹeɪɫmənt// UK //dɪɹˈeɪlmənt// de·rail·ment
n. the act of a train coming off its tracks, or a situation that stops a plan from moving forward. It can also describe a person's mind wandering away from the main topic.
n. the act of a train being forced off its rails; by extension, any event that prevents a process or plan from continuing as intended. Often used metaphorically to describe a loss of focus or a deviation from a logical argument.
The heavy snow caused a derailment on the main line.
A sudden technical failure led to the derailment of the entire project, forcing the team to start over from scratch.
The speaker's tendency to share personal anecdotes often resulted in a derailment of the central thesis, leaving the audience confused about the original argument.