Dismissive avoidant attachment is one of the four adult attachment styles, and it has the most precise definition of the four. In the model researchers built in the early 1990s, attachment patterns sit on two dimensions: how anxious you are about the relationship, and how much you avoid closeness. Dismissive avoidant is the combination of high avoidance and low anxiety. The closeness alarm is sensitive. The abandonment alarm mostly is not.
That combination explains almost everything people find confusing about this style. The dismissive avoidant person is not sitting in fear of losing the relationship, which is why they can go quiet for a day without noticing the silence is loud. What their system tracks is engulfment: the feeling that closeness is about to cost them autonomy, privacy, or self. When that alarm fires, they do not reach for reassurance the way an anxious partner would. They reach for distance.
Dismissive does not mean cruel, and it does not mean indifferent. It means the person tends to dismiss the importance of emotional dependence, their own most of all. Many dismissive avoidant partners are loyal, steady, practical, and affectionate in quiet ways, right up until closeness starts to feel like demand.