PI

UK: /paɪ/ US: /paɪ/ ,(pī)

Inflections of 'pi' (n):

pis

npl (For the Greek letter and mathematical sense)

pies

npl (For the printing sense)

Inflections of 'pi' (v): (⇒ conjugate)

pies

v 3rd person singular

piing

v pres p

pied

v past

pied

v past p

Collins Concise English Dictionary © HarperCollins Publishers::

PI abbreviation

1. Philippine Islands
2. private investigator

WordReference Random House Learner's Dictionary of American English © 2026

pi¹ /paɪ/   n. [countable] , pl. pis. 

1. Linguistics the 16th letter of the Greek alphabet (&Pgr;, π).
2. Mathematics
   • the letter π, used as the symbol for the ratio of the circumference of a circle to its diameter.
   • the ratio itself, a number usually given as 3.141592+.

WordReference Random House Unabridged Dictionary of American English © 2026

pi¹  (pī), n., pl. pis. 

1. Linguistics the 16th letter of the Greek alphabet (II, π).
2. Linguistics the consonant sound represented by this letter.
3. Mathematics
   • the letter π, used as the symbol for the ratio of the circumference of a circle to its diameter.
   • the ratio itself: 3.141592+.

• Greek pî, peî; used in mathematics to represent Greek periphérion periphery
• 1835–45

pi²  (pī), n., pl. pies, v., pied, pi•ing. 
n. 

1. Printing printing types mixed together indiscriminately.
2. any confused mixture;
   jumble.

v.t. 

3. Printing to reduce (printing types) to a state of confusion.
4. to jumble.

Also, pie. 

• origin, originally uncertain 1650–60

PI, 

1. Law personal injury.
2. principal investigator.
3. private investigator.

Pi., 

1. piaster.

Also, pi. 
P.I., 

1. Place Names Philippine Islands.
2. Also, p.i. private investigator.

Collins Concise English Dictionary © HarperCollins Publishers::

pi / paɪ/ noun (pis)

1. the 16th letter in the Greek alphabet (Π, π), a consonant, transliterated as p
2. a transcendental number, fundamental to mathematics, that is the ratio of the circumference of a circle to its diameter. Approximate value: 3.141 592…; symbol: π

Etymology: 18th Century (mathematical use): representing the first letter of Greek periphereia periphery

pi, pie / paɪ/ noun (pies)

1. a jumbled pile of printer's type
2. a jumbled mixture

verb (pies, piing, pied, pies, pieing, pied)(transitive)

1. to spill and mix (set type) indiscriminately
2. to mix up

Etymology: 17th Century: of uncertain origin

pi / paɪ/ adjective

1. 
   short for pious2,