That point in an exercise at which you have so fully fatigued your working muscles that they can no longer complete an additional repetition of a movement with strict biomechanics. You should always take your post-warm-up sets at least to the point of momentary muscular failure, and frequently past that point.
In weight training, the point in your set at which your muscle is so fatigued that you cannot lift the weight anymore. D E J K L M N P Q R S U V W X Y Z
A system has failed if it is unable to deliver correct answers to legitimate requests for any reason, or it delivers correct answers to illegitimate requests, for any reason.