Does Counterspell work on abilities or only on spells?

Counterspell's text reads "Counter target spell." Under CR 114.1, a spell is a card or copy of a card on the stack. Activated and triggered abilities are also placed on the stack, but they are explicitly not spells — they are ability objects (CR 112.1). Because Counterspell targets only spells, it cannot be used to counter an ability.

This distinction matters a great deal in practice. When a player activates, say, the loot ability on Faithless Looting from the graveyard, or when a creature's triggered ability goes on the stack, Counterspell simply cannot target those objects. Only cards or effects that specifically say "counter target activated ability" or "counter target triggered ability" — such as Stifle or Tale's End — can stop abilities on the stack (CR 115.2a).

Similarly, mana abilities are special: they don't even use the stack at all under most circumstances (CR 605.3b), so they cannot be countered by anything, period.

Concrete example: Your opponent attacks with Elvish Visionary and its "enters the battlefield" trigger goes on the stack. You cannot cast Counterspell targeting that trigger — it is an ability, not a spell. You would need Stifle or a similar effect to counter it.

• Spell on stack → Counterspell works (CR 114.1)
• Activated/triggered ability on stack → Counterspell does NOT work (CR 112.1)
• Mana ability → Nothing counters it; it never uses the stack (CR 605.3b)