Palm Of One's Hand
Palm
independent noun
てのひら
手 (て) Hand, Arm
手の甲 (てのこう) Back Of One's Hand
手の平 (てのひら、not on WK) Palm Of One's Hand, Palm
掌 (てのひら) Palm Of One's Hand, Palm
平仮名 (ひらがな) Hiragana
平 has ひら as a secondary kun'yomi reading.
The Palm 掌/手の平 is the flat 平 part of the hand 手