They also usually mean the speed as measured in an inertial frame.
This vacuum-inertial speed is denoted 299,792,458 m/s. Definitions are adopted according to the most accurately known measurement techniques of the day, and are constantly revised.
The short answer is that it depends on who is doing the measuring: the speed of light is only guaranteed to have a value of 299,792,458 m/s in a vacuum when measured by someone situated right next to it.
But let's approach the question by considering its various meanings. Light is slowed down in transparent media such as air, water and glass.
Obviously it would be more natural to attribute those changes to variations in the units of measurement than to changes in the speed of light itself, but by the same token it's nonsense to say that the speed of light is now constant just because the SI definitions of units define its numerical value to be constant.
But the SI definition highlights the point that we need first to be very clear about what we mean by constancy of the speed of light, before we answer our question.(One of the first measurements of the speed of light was derived from observed changes in the timing of the eclipses of Jupiter's moons by Olaus Roemer in 1676.) We could, for example, take the definitions of the units as they stood between 19.Then, the metre was defined as 1,650,763.73 wavelengths of the reddish-orange light from a krypton-86 source, and the second was defined (then as now) as 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of caesium-133.Unfortunately it doesn't mention anything about inertial frames, but you can consider a measurement in an inertial frame to be implied. At the moment you can measure macroscopic distances most accurately by sending out laser light pulses and timing how long they take to travel using a very accurate atomic clock.(The best atomic clocks are accurate to about one part in 10.) It therefore makes sense to define the metre unit in such a way as to minimise errors in such a measurement.The SI definition also assumes that measurements taken in different inertial frames will give the same results for light's speed.This is actually a postulate of special relativity, discussed below.The SI definition makes certain assumptions about the laws of physics.For example, it assumes that the particle of light, the photon, is massless.See the FAQ article Have physical constants changed with time?(Note that the fine-structure constant does change with energy scale, but I am referring to the constancy of its low-energy limit.) Another assumption on the laws of physics made by the SI definition of the metre is that the theory of relativity is correct.