Nuke spell casting is the act of casting a spell that causes ranged magic damage. Nukes generally deal all or most of their damage immediately rather than slowly over time. Since nuke spells cause proportionally high damage early game, they are commonly played and leveled up early in the game.
Nuke spells are often cast in combination or in chain with other abilities, such as a disabling spell or other damage spells.
Some notable examples of this chain or combo abilities are:
Demon Witch - Impale + Voodoo + Finger of Death
Nerubian Assassin - Mana Burn + Impale + Vendetta
Slayer - Light Strike Array + Dragon Slave + Laguna Blade.
To add more damage to their nukes, heroes often buy items such as Dagon and Guinsoo's Scythe of Vyse.
A hero who often uses nuke spells is usually called a Nuker. An example of this would be Zeus, who has 3 nukes. Nukers are typically Intelligence based heroes.
Unfortunately, nukers are also the first target by enemy heroes during early game because they also have the lowest amoung of health points. A good early game for nukers would be Shiva's Guard because it also grants armor to the hero.