on the same wavelength

WordReference Random House Learner's Dictionary of American English © 2025

wave•length /ˈweɪvˌlɛŋkθ, -ˌlɛnθ/   n. [countable]

1. Physics the distance, measured in the direction of a wave, between two successive points in the wave that are characterized by the same phase of oscillation.

Idioms

2. Idioms on the same wavelength, [be + ~] sharing values, ideas, or impulses;
   thinking and acting in agreement or harmony:
   You and I are on the same wavelength about this; we both think the plan needs reworking.

WordReference Random House Unabridged Dictionary of American English © 2025

wave•length  (wāv′lengkth′, -length′, -lenth′), n. 

1. Physics Physics. the distance, measured in the direction of propagation of a wave, between two successive points in the wave that are characterized by the same phase of oscillation.
2. on the same wavelength, in sympathy or rapport:
   We seemed to be on the same wavelength from the moment we met.

Also, wave′ length′. 

• wave + length 1855–60

Collins Concise English Dictionary © HarperCollins Publishers::

wavelength / ˈweɪvˌlɛŋθ/ noun

1. the distance, measured in the direction of propagation, between two points of the same phase in consecutive cycles of a wave
   Symbol: λ
2. the wavelength of the carrier wave used by a particular broadcasting station
3. on someone's wavelength, on the same wavelength ⇒ having similar views, feelings, or thoughts (as someone else)