Aconite - AKA moonkshood or wolfsbane. It's flowers are useful in potion making but it's leaves are toxic. The root of aconite can also be used as a potion ingredient. Used to be found worldwide but now can only be found in wild places.

Antidote - A type of potion which counteracts or reverses any negative effects on the body. Some can also prevent or protect against specific ailments. Although they're not always potions and not specific to a particular technique or phase, the general rule is that they are liquid and consumed orally. 

Artur LeCompt-Voyage - The first wizard/witch to live his life entirely with animals created from magic. He may have believed he had created true life, but these animals did not have free will and had no desire to eat or do anything they weren't supposed to.

Asphodel - Member of the lily family with long and slender leaves. Found in both the magical and muggle worlds. It's powdered roots are used in making potions such as Draught of the Living Death and Wiggenweld potion. 

Astronomy - Study of celestial objects (moons, planets, stars, nebulae, galaxies), the physics, chemistry and evolution of these objects, and phenomena that originate outside the atmosphere of the Earth (supernovae explosions, gamma ray bursts, cosmic background radiation).

Augerey - AKA Irish Phoenix. A thin, mournful looking bird with greenish black feathers and a sharp beak. It was long believed that their cry foretold death, however this was a myth and they actually sing when it is about to rain and so were used as weather forecasters. Their feathers repel ink, making them useless quill feathers. 

Avifors - Avifors. Wand movement: twirl above the object then downwards flick. A Class-A Transfigurative Spell. Transfigures statues and, by extension, other objects into birds. The caster gains control over the bird and can instruct them to perform a simple task. Does not create real life due to Gamp's Law of Elemental Transfiguration.

Awakening Potion - See 'Wideye Potion'