A Spell is the magical ability to alter reality with the spoken word.
A spell is a series of words that when said in a certain order causes changes in the fabric of reality to a varying extent. A spell is a ritual intended to produce a magical effect. In order to be effective, it must be performed by a magical being; a spell recited by a mortal will not work. The act of performing a spell ritual is known as "casting the spell".
Casting a Spell
All spells contain a spoken component, or incantation. An incantation is written and recited as verse. They vary widely in length. They may be as short as two lines or can can be a dozen or more lines in length. Most contain rhymes, although some do not. Typical rhyme schemes include couplets (A,A,B,B) or interlocking couplets (A,B,A,B). Some incantations are in a foreign, ancient or undecipherable language, but still work effectively.
Limitations and Reversal
Typically, a spell can only be reversed by the person who cast it. There are some exceptions. If another person knows the spell, then he or she can reverse it. Once Samantha found the spell Aunt Clara used to accidentally turn Darrin into a chimpanzee, she was able to reverse the spell and restore Darrin to human form.
A counterspell (or anti-spell) negates another spell. Typically this takes the form of a potion and a series of actions to produce the desired results. Samantha performed a counterspell to neutralize a spell Endora cast on Darrin to turn him into a mouse.
Sometimes spells can be ineffective if a witch tries to cast a spell on a more powerful witch. They simply do not have the power to affect a more powerful witch. Samantha cast a spell on her mother to banish her from her honeymoon suite. It failed because Endora is more powerful than Samantha. Likewise, Samantha tried this on her father Maurice and failed.