1.  compression(n = noun.event) compaction, concretion, densification - an increase in the density of something;
is a kind of concentration

2.  compression(n = noun.event) condensation, contraction - the process or result of becoming smaller or pressed together; "the contraction of a gas on cooling"
is a kind of shrinkage, shrinking
has particulars: coarctation, constriction
Derived form verb compress₂

3.  compression(n = noun.act) - encoding information while reducing the bandwidth or bits required;
is a kind of encoding, encryption
has particulars: image compression, mpeg
Antonym: decompression
Derived form verb compress₁

4.  compression(n = noun.act) compressing - applying pressure;
is a kind of press, pressing, pressure
has particulars: squeeze, squeezing, condensation, condensing, compaction, crunch, crush, constriction
Antonym: decompression
Derived form verb compress₂

1.  The act of compressing, or state of being compressed. Johnson. [1913 Webster]

2.  reduction of the space required for storage (of binary data) by an algorithm which converts the data to a smaller number of bits while preserving the information content. The act of compressing {3}.
"Compression may be lossless compression, in which all of the information in the original data is preserved, and the original data may be recovered in form identical to its original form; or lossy compression, in which some of the information in the original data is lost, and decompression results in a data form slightly different from the original. Lossy compression is used, for example, to compress audio or video recordings, and sometimes images, where the slight differences in the original data and the data recovered after lossy compression may be imperceptable to the human eye or ear. The JPEG format is produced by a lossy compression algorithm." [PJC]

compression, n.
1 the act of compressing or being compressed.
2 the reduction in volume (causing an increase in pressure) of the fuel mixture in an internal-combustion engine before ignition.

Etymology:
F f. L compressio (as COMPRESS)

N  conciseness, brevity, the soul of wit, laconism, Tacitus, ellipsis, syncope, abridgment, compression, epitome, monostich, brunch word, portmanteau word, concise, brief, short, terse, close, to the point, exact, neat, compact, compressed, condensed, pointed, laconic, curt, pithy, trenchant, summary, pregnant, compendious, succinct, elliptical, epigrammatic, quaint, crisp, sententious, concisely, briefly, summarily, in brief, in short, in a word, in a few words, for shortness sake, to come to the point, to make a long story short, to cut the matter short, to be brief, it comes to this, the long and the short of it is, brevis esse laboro obscurus fio.

N  compend, compendium, abstract, precis, epitome, multum in parvo, analysis, pandect, digest, sum and substance, brief, abridgment, summary, apercu, draft, minute, note, excerpt, synopsis, textbook, conspectus, outlines, syllabus, contents, heads, prospectus, album, scrap book, note book, memorandum book, commonplace book, extracts, excerpta, cuttings, fugitive pieces, fugitive writing, spicilegium, flowers, anthology, collectanea, analecta, compilation, recapitulation, resume, review, abbreviation, abbreviature, contraction, shortening, compression, compendious, synoptic, analectic, abrege, abridged, variorum, in short, in epitome, in substance, in few words, it lies in a nutshell.

N  contraction, reduction, diminution, decrease of size, defalcation, decrement, lessening, shrinking, compaction, tabes, collapse, emaciation, attenuation, tabefaction, consumption, marasmus, atrophy, systole, neck, hourglass, condensation, compression, compactness, compendium, squeezing, strangulation, corrugation, astringency, astringents, sclerotics, contractility, compressibility, coarctation, inferiority in size, contracting, astringent, shrunk, contracted, strangulated, tabid, wizened, stunted, waning, neap, compact, unexpanded &c (expand), contractile, compressible, smaller &c (small).

compression bandage, compression fracture, compression projectile, image compression, lossy compression, nerve compression