DWDM: Dense Wavelength Division Multiplexing: Wavelength-division multiplexing with signals closely spaced in frequency.
CWDM: Coarse Wavelength Division Multiplexing: Wavelength-division multiplexing with signals broadly spaced in wavelength.
The division of the radio spectrum into broadcast channels made the idea of wavelength-division multiplexing (WDM) obvious to any serious student of communications by the time the laser was invented. But how to divide the optical spectrum was far from obvious. In the early 1980s AT&T picked three widely spaced channels for the first commercial system linking Boston to Washington, GaAlAs lasers at 825 and 875 nm, and an InGaAsP LED at 1300 nm. But single-mode fiber transmission quickly eclipsed its capacity and WDM was largely abandoned.
Invention of the erbium-doped fiber amplifier (EDFA) in the late 1980s revived interest in WDM because it could amplify multiple signals across a range of wavelengths with little crosstalk. The question quickly became how tightly wavelengths could be packed across the 1550 nm erbium-fiber gain band. That required developing new filter technology to slice the spectrum finely. By 2000, channel spacing was down to about 0.4 nm or 100 GHz.
To make systems compatible, the International Telecommunications Union (ITU) defined a standard dense frequency grid spanning the erbium gain band. Each DWDM was 100 GHz wide, with the standard specifying channels in frequency units, such as 193.10, 193.20, and 193.30 THz, although optical optical engineers translated them into wavelengths (1552.52, 1551.72, and 1550.92 nm, respectively).
DWDM was designed for expensive high-performance long-haul systems, but WDM also could enhance capacity of shorter fiber systems, if costs could be cut by using cheaper optics with less-demanding specifications. That led ITU to develop a "coarse" grid, for which they specified CWDM channels in wavelength units, spaced 20 nm apart from 1271 to 1611 nm, used in metro and access networks.
That's the official CWDM grid, but it hasn't stopped designers from multiplexing other combinations of widely-spaced WDM signals, such as cable-television or fiber to the home (FTTH) systems transmitting downstream at 1550 and (sometimes) 1480 nm, and upstream at 1310 nm.
So these divisions of the spectrum do have standard meanings.
Comparison of CWDM and DWDM spacing.