Definition for ir (2 of 7)


Symbol, Chemistry.

Definition for ir (3 of 7)


intelligence ratio.
information retrieval.

Definition for ir (4 of 7)

Definition for ir (5 of 7)


variant of in-2 (by assimilation) before r: irradiate.

Definition for ir (6 of 7)


variant of in-3 (by assimilation) before r: irreducible.

Definition for ir (7 of 7)


immediate reserve.
infantry reserve.
intelligence ratio.
internal revenue. Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

Examples from the Web for ir

British Dictionary definitions for ir (1 of 5)


the internet domain name for


British Dictionary definitions for ir (2 of 5)


the chemical symbol for


British Dictionary definitions for ir (3 of 5)


abbreviation for

(formerly, in Britain) Inland Revenue
Iran (international car registration)

British Dictionary definitions for ir (4 of 5)



a variant of in- 1, in- 2

British Dictionary definitions for ir (5 of 5)


abbreviation for

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Medical definitions for ir


The symbol for the elementiridium
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Scientific definitions for ir (1 of 2)


The symbol for iridium.

Scientific definitions for ir (2 of 2)

[ ĭ-rĭdē-əm ]


A rare, whitish-yellow element that is the most corrosion-resistant metal known. It is very dense, hard, and brittle, and is is used to make hard alloys of platinum for jewelry, pen points, and electrical contacts. Atomic number 77; atomic weight 192.2; melting point 2,410°C; boiling point 4,130°C; specific gravity 22.42 (at 17°C); valence 3, 4. See Periodic Table.

A Closer Look

In 1978 geologist Walter Alvarez discovered a high concentration of iridium in a layer of clay that had formed between the Mesozoic and Cenozoic eras, a period about 65 million years ago during which dinosaurs and many other organisms became extinct. This finding was significant as iridium is rare at Earth's surface (an unusually high concentration is called an iridium anomaly). Most surface iridium is thought to come from dust created when meteors disintegrate in the atmosphere or collide with Earth. Alvarez's father, the physicist Luis Alvarez, suggested that the iridium might have come from the impact of a meteor about 10 km (6.2 mi) across. Such an impact would have caused an enormous explosion, sending huge clouds of dust into the atmosphere. The dust, blocking out the Sun and causing extensive acid rain, would have triggered a worldwide ecological disaster. Many scientists think that such a disaster caused the extinction of the dinosaurs and at least 70 percent of all other species alive at the time, including most of Earth's land plants. Geologists have since found iridium deposits in rocks of a similar age in more than 100 places worldwide. Scientists in the early 1990s identified a large impact crater in the Yucatán peninsula of central Mexico that is the same age as the iridium deposit found by Alvarez. It is 200 km (125 mi) wide and may have been caused by the same impact.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.