na-adjective, noun
Describes physical surfaces or terrain that are flat and without bumps. Also used metaphorically for a smooth, trouble-free path or process.
この道は平坦で歩きやすい。
This road is flat and easy to walk on.
人生は平坦な道ばかりではない。
Life is not always a smooth path.
na-adjective, noun
calm; peaceful; uneventful; trouble-free
Describes a state or situation that is calm, peaceful, and without trouble or excitement. Often used for uneventful periods or a tranquil state of mind.
試験が終わって気分が平坦になった。
After the exam, my mind became calm.
平坦な毎日を過ごしている。
I'm spending uneventful days.
平ら is a more common and casual word for 'flat' or 'level', often used for physical surfaces. 平坦 is slightly more formal and can be used metaphorically.
穏やか means 'calm' or 'gentle', often for weather, personality, or atmosphere. 平坦 in sense 2 emphasizes lack of trouble or excitement rather than gentleness.
From 平 (flat) + 坦 (level, smooth). The kanji 坦 is not a jōyō kanji, which may explain the search-only mixed forms.