Shield spell ("Scildan!") is a powerful form of enchantment used by a sorcerer to protect themselves. It creates a large half-sphere of magical energy that stops the enemy's attack. It's unknown if this spell can only protect its caster from another magical assault or even from other objects, like arrows.
Merlin used this spell to protect himself from Kilgharrah when the angered dragon breathed fire against him. The young warlock displayed his powerful magic with it, being able to stop completely the Great Dragon's attack (Le Morte d'Arthur).
When Merlin engaged Nimueh in a magical duel on the Isle of the Blessed, he attacked the sorceress with a blast of fiery energy. However, being a powerful High Priestess of the Old Religion, Nimueh was able to deflect this attack with her bare hand and a non-verbal spell, quickly creating a magical shield in front of her that vanquished the young warlock's attack (Le Morte d'Arthur).
In the episode Aithusa, Merlin cast a spell ("Þrosm tohweorfe"), related to the element of Air, to protect himself from the poisonous smoke released by a trap door in the Tomb of Ashkanar. With this incantation the young warlock was able to stop the smoke, create an air current and disperse it. It's possible that this enchantment is a type of shield spell.
Morgana tried to cast a shield spell during her magical duel with Emrys. She had just blasted the old sorcerer away with a stunning spell, seemingly incapacitating him, taken the jar with the Fomorroh and started to walk away with it. Merlin was able to gather his strength and cast a powerful whirlwind spell against the Witch. Morgana stretched out her hand toward the cyclone, focusing her energies on it. Merlin pointed the whirlwind too, pushing it forward. The High Priestess was not able to stop or control it, so she lowered her hand with a terrified expression on her face. She was soon hit by Emrys' spell and hurled away, badly wounded (A Servant of Two Masters).
Although not actually a spell, magical beings seem to have a certain natural protection against certain enchantments like stunning spells. When these forms of magic are put on a normal human being, he is always killed or knocked out. On the other hand, sorcerers can be wounded but not easily defeated.
When the young dragon Aithusa, wanting to defend Morgana against the intruders that had entered the mines under the Fortress of Ismere, blew a constant ray of fire against Merlin, the warlock protected himself with a shield spell, deflecting and vanquishing the creature's attack (Arthur's Bane: Part Two).