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moby--moby/docs/sources/terms/image.md
James Turnbull a7b2c4804b Adding User Guide
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Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
2014-06-01 17:34:07 -04:00

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page_title: Images
page_description: Definition of an image
page_keywords: containers, lxc, concepts, explanation, image, container
# Image
## Introduction
![](/terms/images/docker-filesystems-debian.png)
In Docker terminology, a read-only [*Layer*](/terms/layer/#layer-def) is
called an **image**. An image never changes.
Since Docker uses a [*Union File System*](/terms/layer/#ufs-def), the
processes think the whole file system is mounted read-write. But all the
changes go to the top-most writeable layer, and underneath, the original
file in the read-only image is unchanged. Since images don't change,
images do not have state.
![](/terms/images/docker-filesystems-debianrw.png)
## Parent Image
![](/terms/images/docker-filesystems-multilayer.png)
Each image may depend on one more image which forms the layer beneath
it. We sometimes say that the lower image is the **parent** of the upper
image.
## Base Image
An image that has no parent is a **base image**.
## Image IDs
All images are identified by a 64 hexadecimal digit string (internally a
256bit value). To simplify their use, a short ID of the first 12
characters can be used on the command line. There is a small possibility
of short id collisions, so the docker server will always return the long
ID.