A name for the voiceless velar fricative.
The name is German; "laut" means something like "sound", so the idea here is that we mean "the sound in ach". Ach-laut is opposed to ich-laut, the voiceless palatal fricative.
In German these two sounds are allophones of the same phoneme: ach-laut in the environment of back vowels, and ich-laut for front ones.
Note: I don't study German; I learned this on the CONLANG list.