[ oz-moh-sis, os- ]
/ ɒzˈmoʊ sɪs, ɒs- /
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Physical Chemistry, Cell Biology.
  1. the tendency of a fluid, usually water, to pass through a semipermeable membrane into a solution where the solvent concentration is higher, thus equalizing the concentrations of materials on either side of the membrane.
  2. the diffusion of fluids through membranes or porous partitions.Compare endosmosis, exosmosis.
a subtle or gradual absorption or mingling: He never studies but seems to learn by osmosis.


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Origin of osmosis

1865–70; Latinized form of now obsolete osmose osmosis, extracted from endosmose endosmosis, exosmose exosmosis <French, equivalent to end-end-, ex-ex-2 + Greek ōsm(ós) push, thrust + French -ose-osis


Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2022


What is osmosis?

Osmosis is the movement of a fluid from an area with a low concentration of a solute through a semipermeable membrane to an area with a high concentration of a solute.

A solute is something that dissolves in something else, known as a solvent, creating a solution. In sugar water (a solution), the sugar is the solute that dissolves in the water, the solvent. Concentration is the measurement of how much solute is in a solvent. Semipermeable means that only certain things can pass through something. In osmosis, only the solvent (usually water) can pass through the membrane.

The process of osmosis works like this. Let’s say we have a glass that is separated down the middle with a plastic divider with tiny holes in it that only water can move through. First, we fill the glass with water. Then, we add 5 grams of salt to one half of the glass and 7 grams of salt to the other half. What happens?

Over time, water will flow from the half with 5 grams of salt (the lower concentration) into the half with 7 grams of salt (the higher concentration). Water will stop flowing (or flow incredibly slowly) when the two halves have about the same amount of salt in each side.

Osmosis is a natural process that a solvent will want to do. Both plants and animals use osmosis to absorb water into their highly concentrated cells. Without osmosis these living things would be more likely to suffer from dehydration.

Why is osmosis important?

The first records of the term osmosis come from around 1865. It is the Latinized form of the obsolete noun osmose, which comes from the Greek word osmos, meaning “push” or “thrust.” In osmosis, a solvent “pushes” or “thrusts” through a membrane.

Water is the most common solvent involved in osmosis because of how prevalent and important it is. Importantly, osmosis is a passive process, which means cells don’t have to spend energy to perform it. Water will naturally flow into the cells.

Osmosis is a sort of fluid version of a process known as diffusion, in which something of high concentration moves to an area of low concentration. In the case of osmosis, a solvent with less stuff in it is highly concentrated so it wants to move through a barrier to help fill an area with more solute that is taking up space and making the solvent around it less concentrated.

Did you know … ?

Osmosis is used outside of science to refer to any passive flow or absorption. For example, we might describe a student as learning “through osmosis,” meaning they don’t actively study but gradually absorb the knowledge over time.

What are real-life examples of osmosis?

Osmosis is a process that is commonly learned by students new to biology or chemistry.


What other words are related to osmosis?

Quiz yourself!

True or False?

During osmosis, water flows from an area with a high concentration of solute through a membrane into an area with a low concentration of solute.

How to use osmosis in a sentence

British Dictionary definitions for osmosis

/ (ɒzˈməʊsɪs, ɒs-) /

the passage of a solvent through a semipermeable membrane from a less concentrated to a more concentrated solution until both solutions are of the same concentration
diffusion through any membrane or porous barrier, as in dialysis
gradual or unconscious assimilation or adoption, as of ideas

Derived forms of osmosis

osmotic (ɒzˈmɒtɪk, ɒs-), adjectiveosmotically, adverb

Word Origin for osmosis

C19: Latinized form from osmose (n), from Greek ōsmos push, thrust
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

Scientific definitions for osmosis

[ ŏz-mōsĭs ]

The movement of a solvent through a membrane separating two solutions of different concentrations. The solvent from the side of weaker concentration usually moves to the side of the stronger concentration, diluting it, until the concentrations of the solutions are equal on both sides of the membrane.♦ The pressure exerted by the molecules of the solvent on the membrane they pass through is called osmotic pressure. Osmotic pressure is the energy driving osmosis and is important for living organisms because it allows water and nutrients dissolved in water to pass through cell membranes.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for osmosis

[ (ahz-moh-sis, ahs-moh-sis) ]

The seeping of a fluid through a seemingly solid barrier, such as a cell wall or a rubber sheet. When the concentration of the fluid is the same on both sides of the barrier, osmosis stops.

notes for osmosis

Informally, “osmosis” is the process by which information or concepts come to a person without conscious effort: “Living in Paris, he learned French slang by osmosis.”
The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.