noun

a line on the surface of something along which it has split without breaking apart.

a hairline crack down the middle of the glass

split,fissure,crevice,break,fracture,rupture,breach,rift,cleft,slit,chink,gap,cranny,interstice,crazing

verb

break or cause to break without a complete separation of the parts.

the ice all over the bog had cracked

split,fracture,fissure,rupture,break,snap,cleave,craze