flap

Compared meaning: to move up and down or side to side (verb).

比较义项：摆动（动词）。

• wave

  A smooth, gentle up-and-down or side-to-side motion, often a deliberate hand gesture used to greet, signal, or say goodbye to someone.

  指轻柔、平缓的上下或左右摆动，常指用手做出的有意动作，用来打招呼、示意或告别。

  She waved happily from the platform as the train pulled away.

  她在站台上开心地挥手，目送火车驶离。

• flap 本词

  A quick, often noisy up-and-down or back-and-forth movement, typically of wings, flags, sails, or loose fabric caught by wind or motion.

  指急促、常伴有声响的上下或来回摆动，多用于翅膀、旗帜、船帆或被风吹动的松散布料。

  The goose flapped its wings hard before finally lifting off the ground.

  那只鹅用力拍打翅膀，才终于飞离地面。

Wave and flap both mean to move back and forth. Wave is smooth and gentle, a deliberate hand gesture. Flap is quick and noisy, describing wings, flags, sails, or loose cloth in the wind. Not compared: wave as 'ocean wave', or flap as 'envelope flap'.

wave 和 flap 都表示“摆动”，但语气和场景不同：wave 强调轻柔、有意为之的挥动，常用于打招呼或示意；flap 强调急促、常伴有声响的扇动，多用于翅膀、旗帜或随风摆动的物体。这里不比较 wave 作名词表示“波浪”的用法，也不比较 flap 作名词表示“信封盖”或“慌乱”的用法。