SI
Abbreviation with 7 meanings
| Stands for | Suicidal ideation |
|---|---|
| Meanings | 7 |
| Categories | Medical, Units & Measurements, Science & Engineering, Geography & Places, Texting & Internet Slang |
| Also written | Si |
SI has seven meanings. The most common are Suicidal ideation (mental health & psychiatry) and International System of Units (metric and SI unit); it can also stand for Silicon, Sacroiliac (joint), Slovenia, Self-insert and Staten Island.
Meanings of SI at a glance
| # | SI stands for | Context |
|---|---|---|
| 1 | Suicidal ideation | Medical › Mental Health & Psychiatry |
| 2 | International System of Units | Units & Measurements › Metric and SI Unit |
| 3 | Silicon | Science & Engineering › Chemical Element |
| 4 | Sacroiliac (joint) | Medical › Anatomy |
| 5 | Slovenia | Geography & Places › Country Code |
| 6 | Self-insert | Texting & Internet Slang › Fandom and Fan Fiction |
| 7 | Staten Island | Geography & Places › City and Neighborhood |
1. Suicidal ideation
Medical · Mental Health & Psychiatry Abbreviations
Thoughts about, considering or planning suicide, ranging from passive wishes to be dead to active thoughts with a specific plan and intent. Clinicians assess frequency, plan, intent, access to means, prior attempts and protective factors to estimate risk and build a safety plan.
Example: Endorses passive SI, no plan or intent, no firearms at home; safety plan completed, 988 reviewed.
Risk assessment templates commonly record "denies SI/HI". The 988 Suicide & Crisis Lifeline, launched in July 2022, is the US three-digit number for mental health crises. Asking directly about suicide does not increase risk. In anatomy and orthopedics SI means sacroiliac, as in the SI joint.
2. International System of Units
Units & Measurements · Metric and SI Unit Abbreviations
The modern metric system, used officially by nearly every country and universally in science, medicine and international trade. It defines seven base units (meter, kilogram, second, ampere, kelvin, mole and candela), 22 named derived units and 24 decimal prefixes.
Example: All measurements in the lab report must be given in SI units with appropriate prefixes.
The initials come from French, Systeme international d'unites, adopted by the 11th General Conference on Weights and Measures (CGPM) in 1960. Since the 2019 redefinition, every base unit is tied to an exact constant such as the speed of light or the Planck constant. The United States, Liberia and Myanmar are often cited as the only countries that have not made it the main system for everyday use.
3. Silicon (Si)
Science & Engineering · Chemical Element Abbreviations
Silicon (element 14) is a metalloid second only to oxygen in abundance in Earth's crust, where it occurs as silica and silicate minerals in sand, quartz and granite. Ultrapure crystalline silicon is the base material of nearly all computer chips and most solar cells, which gave California's Silicon Valley its name.
Example: The wafers are float-zone Si, 200 mm in diameter, with resistivity above 1,000 ohm-cm.
Jöns Jacob Berzelius is usually credited with isolating silicon in 1823. The name comes from Latin silex (flint), with the -on ending proposed by Thomas Thomson to group it with carbon and boron. Do not confuse silicon with silicone, a family of flexible polymers with a silicon-oxygen backbone used in sealants, bakeware and implants.
4. Sacroiliac (joint)
Medical · Anatomy Abbreviations
Joint between the sacrum and each ilium of the pelvis, transmitting the weight of the upper body to the legs with very little movement. SI joint dysfunction or inflammation (sacroiliitis) is a common source of low back and buttock pain and an early feature of ankylosing spondylitis.
Example: Positive FABER and thigh thrust on the right; SI joint injection scheduled.
Bilateral sacroiliitis is a hallmark of axial spondyloarthritis, and MRI shows inflammation years before X-ray changes appear. Diagnostic injections of local anesthetic help confirm the joint as the pain source. SI also abbreviates the International System of Units, unrelated to anatomy.
5. Slovenia
Geography & Places · Country Code Abbreviations
Slovenia's code comes from Slovenija; Ljubljana is the capital and SVN the alpha-3 code. In 2007 it became the first former communist country to adopt the euro.
Example: Slovenian VAT numbers start with SI followed by eight digits.
Slovenia (SI) and Slovakia (SK) are often confused, even by diplomats. SI also means the International System of Units. The language code sl means Slovenian, while SL is Sierra Leone. The IOC uses SLO.
6. Self-insert
Texting & Internet Slang · Fandom and Fan Fiction Abbreviations
Character based on the author, or the author themselves, placed into a fictional world; also a whole genre of fics built on that premise, such as waking up inside a favorite series.
Example: It's an SI fic where the author wakes up as a first-year at Hogwarts.
Self-insert stories are popular in fan fiction and webnovels, where they overlap with the "isekai" genre of being transported to another world. Critics sometimes dismiss them as wish fulfillment, but many SI fics are celebrated for clever use of canon knowledge. Related to reader-insert fiction using Y/N.
7. Staten Island
Geography & Places · City and Neighborhood Abbreviations
Least populous of New York City's five boroughs, connected to Manhattan by the free Staten Island Ferry and to Brooklyn by the Verrazzano-Narrows Bridge. Locals and city agencies often write SI.
Example: The SI Ferry runs 24 hours a day and costs nothing.
Staten Island residents voted in 1993 to secede from New York City, but the state legislature never acted on it. SI also means the International System of Units and Slovenia.
Related abbreviations
More mental health & psychiatry abbreviations
- SA – Suicide attempt
- SAD – Seasonal affective disorder
- SBIRT – Screening, Brief Intervention, and Referral to Treatment
- SCZ – Schizophrenia
- SIB – Self-injurious behavior
- SLD – Specific learning disorder
- SMI – Serious mental illness
- SUD – Substance use disorder
See all 100 mental health & psychiatry abbreviations →
More metric and SI unit abbreviations
- q – Quecto- (10^-30)
- R – Ronna- (10^27)
- rad – Radian
- S – Siemens
- sr – Steradian
- Sv – Sievert
- T – Tera- (trillion)
- um – Micrometer (micron)
See all 64 metric and SI unit abbreviations →
This entry is for general education and is not medical advice. Abbreviation use varies between institutions; always confirm in context.