What is conditioning?

In one line

In bodybuilding, conditioning means how clearly each muscle's outline shows because there is very little body fat.

What is conditioning?
What 'great conditioning' means on stage
In bodybuilding you often hear 'his conditioning is great' or 'her conditioning is off'. It sounds like being in good condition, as in health or mood, but it means something else. Here is what part of the body on stage the word is about.
In one line
How sharp the muscles look with very little fat
Conditioning means how clearly each muscle's outline shows because there is very little body fat. Researchers also use the word to mean 'low body fat' or 'muscle definition'.
Background: fat under the skin
The fat layer between skin and muscle
Just under the skin there is a layer that joins the skin to the muscles, and this layer holds a lot of fat. This fat is called 'subcutaneous fat'. A body prepared for a contest has so little of it that none can be seen.
Definition
How clearly each muscle can be told apart
The International Fitness and Bodybuilding Federation (IFBB) bodybuilding rules look at muscle quality as well as size, and 'definition' is one part of quality. Definition is how clearly each muscle can be seen as separate through the skin. 'Good conditioning' usually means high definition.
Water under the skin
Water is thought to blur definition
Competitors believe that water building up under the skin blurs the muscle outlines and makes conditioning worse. So they call a body with sharp outlines and no watery layer 'dry', and see it as a good thing.
Too much fat
The IFBB lists it as a reason to bar a competitor
The IFBB bodybuilding rules list a body with too much fat as a reason for the head judge to stop a competitor from competing or to disqualify him. A sticking-out belly is on the same list.
Each division wants a different level
Men's physique and bikini want a less extreme look
The IFBB men's physique rules want low fat but a 'softer' and 'smoother' look than bodybuilding. The bikini rules say very low body fat and a dried-out look should be marked down. The conditioning that bodybuilding rewards loses points in these divisions.
How it is built
Losing fat over several months
Conditioning is built by losing fat while keeping muscle over several months of contest preparation. Researchers think it is best to reach the target conditioning a few weeks before the contest. This is so that in the last week, 'peak week', competitors focus on the body's water instead of fat.
A common misunderstanding
Does good conditioning mean being in good shape?
No. In the dictionary, 'condition' means your state of health or mood, but conditioning on stage is about how the body looks. A review of research on men found that eating very little for months, until body fat neared its lowest limit, came with muscle loss and mood problems.
Summary
How sharp the muscles look with very little fat
Conditioning means how clearly each muscle's outline shows because there is very little body fat. Good conditioning is about looks; it does not mean the body is healthy. Related terms: peak week, striations, water cut.