An action required to activate spells.
Includes one step (single-action) spells activated just by running magical energy, one bar spells that anchor something within the body, and instant contract (ten count) spells, which are simple rituals composed of ten or more bars.
Rin's Gandr Shot and Rider's mystic eyes are both classified as single-action, while the long incantations used by Shirou and Archer exceed five bars, making them closer to ten count.
Caster's incantations are technically one bar, but since their activation is instantaneous they're actually as fast as or even faster than single-action. However, despite their brevity, the power of Caster's spells is equivalent to magecrafts of over five bars. For proper magi, you could call her an insurmountable opponent.
By the way, because Caster's spells themselves are “divine words,” they can't really be described in terms of steps or bars.
Thus, the conditions outlined in Magic Resistance ranks B and C do not apply.

Fate/side material: Fate Encyclopedia