Arithmancy is divination with numbers.
Or, as I like to make it, they see the numbers in everything.
Speed, Distance, Velocity, Acceleration, Force, Angles.
The could calculate the spread of shrapnel from a grenade exploding or tell where an attack would land.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather