SI

Eso es el Amor, Si Señor

• SI, the abbreviation of Système international. (Système international d'unités)
• SI, the acronym of Latin Sancte Ioannes, the phrase ending the hymn Ut queant laxis from earlier words of which the other notes of solfège were derived.
• si (plural sis music) A syllable used in solfège to represent the seventh note of a major scale.
• SI, (anatomy, medicine) Initialism of sacroiliac (anatomy) relating to the sacrum & ilium, or to the region of the lower back where they are located.

Sacroiliac joint, Gray 242

The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is strong, supporting the entire weight of the upper body. It is a synovial plane joint with irregular elevations and depressions that produce interlocking of the two bones.

The human body has two sacroiliac joints, one on the left and one on the right, that often match each other but are highly variable from person to person.

Sacrum (anatomy) A large triangular bone at the base of the spine, located between the two ilia (wings of the pelvis) and formed from vertebrae that fuse in adulthood.

From Latin os sacrum (“holy bone”), a calque of Ancient Greek ἱερὸν ὀστέον (hieròn ostéon). Apparently so-called either because the sacrum was the part of the animal offered in sacrifice or because of a putative belief that it is where a person's soul resides. A third explanation is that the term is a mistranslation of Ancient Greek ἱερὸν (hieròn), which has two meanings: “holy, sacred”, and “big” — big being a more appropriate description of the sacrum — but compare.

SI base units
Symbol	Name	Quantity
s	second	time
m	metre	length
kg	kilogram	mass
A	ampere	electric current
K	kelvin	thermodynamic temperature
mol	mole	amount of substance
cd	candela	luminous intensity
SI defining constants
Symbol	Name	Exact value
ΔνCs	hyperfine transition frequency of Cs	9192631770 Hz
c	speed of light	299792458 m/s
h	Planck constant	6.62607015×10−34 J⋅s
e	elementary charge	1.602176634×10−19 C
k	Boltzmann constant	1.380649×10−23 J/K
NA	Avogadro constant	6.02214076×1023 mol−1
Kcd	luminous efficacy of 540 THz radiation