CaO2	oxygen content in arterial blood
[Hb]	hemoglobin concentration in blood
HbO2	oxygenated hemoglobin
dHb	deoxygenated hemoglobin
SO2	oxygen saturation in blood vessels, arteries or veins
SaO2	arterial oxygen saturation
SpO2	arterial oxygen saturation measured by pulse oximetry
PaO2	arterial oxygen tension
SvO2	oxygen saturation in venous blood
StO2	oxygen saturation in tissue blood
SjO2	oxygen saturation in the jugular vein
SavO2	weighted mean of oxygen saturation in blood in the jugular vein and in a systemic artery
µa	absorption constant
µs	scattering coefficient
µs’	reduced scattering coefficient
g	anisotropy
εO	extinction coefficients of HbO2
εD	extinction coefficients of dHb
ε	the mean extinction coefficient of blood in the tissue vessels, arteries and veins
C	hemoglobin concentration
I0	incident light intensity
It	transmitted light intensity
d	emitter-detector distance
l	mean optical path-length
DPF	differential pathlength factor
G	light-loss due to tissue scattering
OD	optical density
PPG	photoplethysmography
ID	The maximal value of the PPG pulse
IS	The minimal value of the PPG pulse
AC	the difference between the maximal and minimal values of the PPG pulse
DC	the mean light transmission during the pulse
λ	wavelengths
ΔC	the maximal hemoglobin concentration increase in the tissue during systole
R	ratio of ratios
NIRS	Near infrared spectroscopy
TOI	tissue oxygenation index
rSO2	regional oxygen saturation
CytOx	cytochrome oxidase
IR(ρ,t)	the intensity of the transmitted/reflected light in NIRS
ρ	the distance from the light-source
t	time
CW	constant wave/intensity
fNIRS	functional NIRS
fNIRI	functional near infrared imaging
DOT	diffuse optical tomography
PtO2	oxygen partial pressure in a tissue
TBI	traumatic brain injury
CAR	cerebral autoregulation
CBF	cerebral blood flow
MAP	mean arterial blood pressure