断掌是指手掌纹感情线与智能线重合成一直线，紧握手指，可见一条线贯穿整个掌面而过，中间没有中断。相对断掌而言，非断掌掌纹，感情线与智能线是不会相交重合的。也有一些是假断掌的，两线虽有交错，但并未完全重合。有双手均为断掌，也有只有左或右手为断掌。

古时认为“男断掌掌朝纲，女断掌守空房”，亦曰“男人断掌千金享，女人断掌害爹娘”。因此如家有断掌女都会视为不吉利，而娶妻也会忌讳娶断掌女，认为断掌女克夫，会守空房。现代的算命者则认为，断掌的人只是个性固执，神经过敏易和人发生争执，断掌的人常会伴有神经过敏或神经衰弱，胆固醇或中性脂肪过多等疾病，年长后可能会减弱，女断掌个性好强，不喜求人，常会独来独往，人缘差，可能有月经痛和生殖系统发育不健全。

In humans, a single transverse palmar crease is a single crease that extends across the palm of the hand, formed by the fusion of the two palmar creases (known in the pseudoscience of palmistry as the "heart line" and the "head line") is found in half of people with Down Syndrome. It is also found in 10% of the general population.