tonfedd

Welsh

ton +‎ -fedd

IPA^(key): /ˈtɔnvɛð/

tonfedd f (plural tonfeddi)

wavelength
- 2020 May 12, Dr Glyn Morris, “Chwalu'r chwedlau am coronafeirws”, in BBC Cymru Fyw‎^[1]:
  Ond os ofynnwch (sic) i ffisegydd, bydden nhw'n dweud bod tonfedd y lliw coch bob amser yn bresennol, ac felly mae'r datganiad yn wir.
  But if you ask a physicist, they would say that the wavelength of the color red is always present, and so the statement is true.

Welsh mutation
radical	soft	nasal	aspirate
tonfedd	donfedd	nhonfedd	thonfedd
Note: Some of these forms may be hypothetical. Not every possible mutated form of every word actually occurs.