1. (n.) A long, hollow vessel, generally for holding water or other liquid, especially one formed by excavating a log longitudinally on one side; a long tray; also, a wooden channel for conveying water, as to a mill wheel.

2. (n.) Any channel, receptacle, or depression, of a long and narrow shape; as, trough between two ridges, etc.

adit aerospace aerosphere air hole air pocket airspace alveolation alveolus amplitude antinode antrum aqueduct armpit basin billow bore bottom bottom glade bottoms bowl breakers bump burrow canal canalization canalize carve cavity ceiling chamfer channel chisel chop choppiness chopping sea chute comb comber concave concavity conduit corrugate course crack crater crest crimp crosswind crypt cup cut dado dale de Broglie wave dell delve depression diffraction dig dig out dike dingle dip dirty water ditch dredge drill drive duct eagre eaves trough egress electromagnetic radiation electromagnetic wave empty space engrave entrance entrenchment excavate exit favorable wind flute fog fold follicle fosse frequency frequency band frequency spectrum front funnel chest furrow gap gash gill glen goffer gouge gouge out gravity wave groove ground swell grove grub guide guided wave gully gutter ha-ha head wind heave heavy sea heavy swell high-pressure area hole hollow hollow shell in phase incise ingress interference intervale ionosphere jetstream kennel lacuna lift light longitudinal wave lop low-pressure area lower lunar rill mechanical wave mine moat node out of phase overcast pass passage passageway peak penstock pentrough period periodic wave pit pleat plow pocket popple punch bowl quarry rabbet radio wave ravine ray reinforcement resonance resonance frequency riffle rifle ripple rise roll roller rough water roughness rut sap scend scoop scoop out score scrabble scrape scratch sea seismic wave send shell shock wave shoot shovel sink sinus slit socket sound wave soup space spade strath stratosphere streak striate substratosphere sunk fence surf surface wave surge swell tail wind tidal bore tidal wave tide wave transverse wave trench tropopause troposphere troughing troughway tsunami tunnel turbulence undulation vale valley visibility visibility zero vug wadi water wave wave wave equation wave motion wave number wavelength wavelet way white horses whitecaps wrinkle