In an ideal way; perfectly.
"The garden would ideally be planted in early spring to ensure all the flowers bloom together."
In plain English: Ideally means exactly how you wish something to be, even if it doesn't always happen that way.
"Ideally, we should finish the project before Friday so everyone has time to review it."
Ideally comes from combining the word ideal with the suffix -ly to form an adverb. It entered English by adding a common grammatical ending to describe doing something in accordance with an ideal.