* Base: A base is a substance that can accept protons (H+) or donate hydroxide ions (OH-) in solution. Magnesium oxide, when dissolved in water, forms magnesium hydroxide (Mg(OH)2), which can neutralize acids.

* Alkali: An alkali is a specific type of base. It is a soluble base, meaning it dissolves readily in water and produces hydroxide ions (OH-) in solution. While MgO is a base, it is not very soluble in water, so it doesn't fit the definition of an alkali.

In summary: Magnesium oxide is a base due to its ability to react with acids and form salts. However, its limited solubility in water prevents it from being classified as an alkali.