na-adjective, noun
Describes a surface that is even and flat, or a state of mind or atmosphere that is calm and untroubled. Often used in literary or slightly formal contexts.
この土地は平らかで、家を建てるのに適している。
This land is level and suitable for building a house.
彼の心はいつも平らかだ。
His mind is always calm.
平ら is the more common word for 'flat' or 'level' in everyday speech. 平らか is slightly more formal or literary, and can also imply peacefulness.
穏やか is the standard word for 'calm' or 'gentle' in modern Japanese. 平らか is less common and carries a nuance of flatness or evenness in addition to calmness.
Derived from the adjective stem 平ら (たいら, 'flat') + suffix か, forming a classical adjectival noun. The exact historical development is uncertain.