Sp
Sp has 3 meanings
Spell power
World of Warcraft (WoW) players use SP as shorthand for "spell power." This character attribute (which can be buffed by items and potions) increases the power of both damaging and healing spells.
A character's spell power is not directly applied to damaging and healing spells. Instead, each spell has a unique "spell power coefficient," which determines how much of a boost your spell power gives it. Thus, while your spell power might add 30 dmg to one spell, it may add only 15 dmg to another.
Example
How do I increase my mage's SP?
You'll want to find gear that boosts spell power. Abyssal items will do the trick
How well do you know your texting slang?
Those who are unsure they've spelled a word correctly may use (sp) to say so. You might also see this abbreviation written as (sp?), to emphasize its author's uncertainty.
For example, while discussing politics, your friend Jake might send a message that reads "He advocates for the most ineagleitarian (sp?) policies but people keep voting for him." The (sp?) in that message denotes that Jake is not sure he spelled inegalitarian correctly. (He didn't.)