pi
pi — noun
1. the fixed number, about 3.14, produced by dividing a circle's outer edge length
the fixed number, about 3.14, produced by dividing a circle's outer edge length by the straight line through its centre; it is basic in circle calculations.
Ms. Lin wrote pi beside 3.14 on the whiteboard.
pi is about 3.14
Using pi, Aaron found the pond's area for the garden plan.
use pi to calculate area
The phone app stores pi to many digits for science homework.
Without pi, Maya could not measure the bike wheel correctly.
- value of pi
- calculate with pi
- pi to many digits
- pi symbol
- circle constant
technical term for the same mathematical number
- Archimedes' constant
formal mathematical name, much less common in school use
- 3.14
a short rounding, not the exact value
Grammar Patterns
value of pi
use pi to calculate + circle measurement
pi is about 3.14
Usage Note
Often written with the Greek symbol π or with the Roman spelling pi in English text. Distinguish from sense 2: here the word names the number, not the alphabet letter itself.
Common Mistakes
2. the name or symbol for the sixteenth character in the Greek alphabet.
the name or symbol for the sixteenth character in the Greek alphabet.
In class, Dr. Hsu asked everyone to copy pi and sigma.
write/copy pi
The old book marks each chapter with a large red pi.
On the chart, pi comes after omicron and before rho.
pi comes after omicron
Lena wore a badge shaped like pi at the language fair.
- Greek letter pi
- capital pi
- small pi
- copy pi
- Greek character
broad term for any symbol in the Greek alphabet, not only pi
- symbol
much wider in meaning and only works when the context already makes pi clear
Grammar Patterns
the letter pi
write/copy pi
pi comes after omicron
Usage Note
Usually appears when people are naming or writing Greek symbols. Distinguish from sense 1, where pi names the mathematical value built from this letter.
Common Mistakes
3. a short way of writing private investigator, a detective hired by private people
a short way of writing private investigator, a detective hired by private people or companies rather than by the police.
The worried husband hired a PI to follow the missing driver.
hire a PI
By noon, the PI had already checked three hotels near the station.
At the motel, the PI paid cash for a room near Leo.
The film ends when the PI shows the judge new photos.
the PI + past verb
- hire a PI
- PI report
- PI agency
- the PI checked
- private investigator
the full form; PI is the short version
- detective
broader and can also mean a police detective
- sleuth
informal and often playful
Grammar Patterns
hire a PI
the PI + past verb
PI report
Usage Note
Usually written as PI in capitals in stories, ads, and receipts. Distinguish from sense 4: this meaning is about detective work, not academic research.
Common Mistakes
4. a short way of writing principal investigator, the main researcher who is in cha
a short way of writing principal investigator, the main researcher who is in charge of a science or engineering project.
Dr. Chen signed the grant form as the study's PI.
the PI of a study
Before hiring students, the PI checked the lab budget twice.
At the meeting, our PI explained why the robot tests stopped.
our PI said + clause
The hospital named Sara Kim the PI for the vaccine project.
name somebody the PI
- study PI
- named the PI
- PI on the grant
- our PI
- principal investigator
the full form used in formal research writing
- lead researcher
plain description of the person in charge
- project lead
broader and can be used outside scientific research
Grammar Patterns
the PI of a study
name somebody the PI
our PI said + clause
Usage Note
Usually written as PI in capitals on grant forms, lab documents, and project emails. Distinguish from sense 3: this PI runs research rather than investigations for clients.
Common Mistakes
5. a short way of writing profitability index, a finance number used to judge how s
a short way of writing profitability index, a finance number used to judge how strongly a project is expected to earn money.
The report says the solar plan has a PI above one.
PI above one
When costs rose, the PI fell below the bank's target.
PI below one
Our teacher used PI to compare two factory projects.
calculate PI for a project
In the spreadsheet, PI appears next to payback time and risk.
- PI above one
- PI below one
- project PI
- calculate PI
- profitability index
the full finance term; PI is the standard abbreviation
- investment ratio
a looser descriptive phrase rather than the fixed label
Grammar Patterns
PI above one
PI below one
calculate PI for a project
Usage Note
Common in finance reports beside numbers such as 1.1 or 0.9. A value above 1 usually suggests the project is worth more than it costs.
Common Mistakes
6. a short way of writing professional indemnity, business insurance that helps pay
a short way of writing professional indemnity, business insurance that helps pay when a client says professional work caused a loss or problem.
The architect renewed her PI before signing the hotel contract.
renew PI
Without PI, the small design firm could not bid today.
The broker advised Malik to raise his PI after two claims.
buy PI
The firm's PI paid the legal bill after a client complaint.
PI insurance
- buy PI
- renew PI
- PI insurance
- raise PI cover
- professional indemnity insurance
the full form used in contracts and policy documents
- errors and omissions insurance
common American term for a similar kind of cover
- liability cover
broader and can include other kinds of protection
Grammar Patterns
buy PI
renew PI
PI insurance
Usage Note
Usually written as PI or PI insurance in business documents. Object is normally a policy bought by people who give paid professional advice or design work.
Common Mistakes
7. a short way of writing personal injury, harm to a person's body or mind, especia
a short way of writing personal injury, harm to a person's body or mind, especially in a legal case against someone else.
The lawyer moved Nina's PI case to the county court.
PI case
On the claim form, PI covers both pain and lost wages.
PI claim
After the crash, the insurer opened a PI file for Jorge.
PI file
The firm offers PI advice after workplace accidents.
- PI case
- PI claim
- PI file
- PI lawyer
- personal injury
the full legal term; PI is the common short form in files and ads
- bodily harm
focuses more on physical damage and less on the legal case
- injury claim
names the request for compensation rather than the harm itself
Grammar Patterns
PI case
PI claim
PI file
Usage Note
Often appears before words like case, claim, file, or lawyer in legal English. Distinguish from sense 6: this PI is the injury itself or the related legal matter, not the insurance policy.
Common Mistakes
Synonym discussion
Sense: noun/1
For noun/1, pi is the standard name for one exact mathematical constant. Ratio is much wider: it can describe any comparison between two amounts, not only the circle relationship. Constant is also broader, because math has many fixed numbers besides pi. Decimal is only a way to write a number, so 3.14 is a decimal approximation of pi, not a different name for it. Tau is the nearest rival term in modern math talk, but it names a different constant equal to 2 pi, so it cannot replace pi in ordinary formulas. A helpful memory line is: pi is the usual circle number in school math, while tau belongs to a smaller alternative system.
Etymology
English took pi from the name of the Greek letter pi. That letter later became the usual mathematical symbol for the circle constant in sense 1, and modern English also writes it as PI when it is a short form for several professional and business terms.