

Someone's subjective idea
(may be based on a formula applied objectively, but the choice to use the formula was someone's subjective idea. Money is not a factor. 
Objectively observable as behavior in the market.
Money is a key factor. "Demand" is also called "effective demand," because it's expressed only by spending money. 
Example of "need": HillBurton hospitalbuildingsubsidy program used 4.5 bed/1000 pop formula (more in rural areas). On the other hand, DHEC's "need" formula for hospital beds is based on historical ratios of utilization to population. This makes it a demand measure.
Health plans that focus on need and ignore demand will face under or
overutilization of service capacity. If one believes quantity demanded
is too little (e.g. indigents) or too much (e.g. overuse of emergency room)
relative to need, then quantity demanded must be manipulated,
by changing price or other costs to buyer, or
by changing demand through marketing or demarketing.
Demand is a relationship between price and quantity. We can study
how high the demand is  the position of the demand curve
 and
how elastic (responsive to price changes) demand is 
the steepness and shape of the curve.
Elasticity of demand is a measure of the responsiveness of the demanded quantity to price changes.
The elasticity of demand for something is:
For instance, If price goes up 1% and as a result sales fall 2%, the
elasticity is 2%/1%=2.
Elasticity, as a measure of responsiveness to price change, is an alternative
to the slope, which would be
Example of calculation (from Phil Jacobs' book):
Health Dept. charge per shot  Shots done per month 
$3  1200 
$3.25  1150 
Slope of demand for tests is
50 tests / $0.25 = 200 tests per $ of price change.
I used 1175 and $3.125 in the elasticity formula. These are averages
of the before and after quantity and price levels. 1175 = (1150+1200)/2.
$3.125 = ($3.00 + $3.25)/2. The elasticity calculated this way is
called the arc elasticity.
Notice how the slope has units (tests per dollar), but the elasticity does not. The elasticity has no units because the numerator is tests/tests and the denominator is $/$. All the units cancel out, leaving a unitless number. This means you get the same answer regardless of what units you use. This lets you compare elasticities of totally different products, like medical care, petroleum, and videocassette rentals.
One interpretation of elasticity of demand:
It's the percentage change in quantity demanded that comes from a 1%
change in the price.
Here's another way to interpret elasticity of demand:
Suppose you raise the price of something you're selling.
Will you make more money? The answer depends on the elasticity of
demand.
Elasticity (absolute value, ignoring the minus sign)  If price goes up,  Demand is categorized as: 
> 1  spending goes down.  Elastic 
= 1  spending stays the same.  Unitary elasticity 
< 1  spending goes up.  Inelastic 
What are examples of products or services whose demand is elastic? Inelastic?
Hot tip: If you want to impress economists, say: "That depends on the elasticity, doesn't it?"
The small elasticities observed for medical care demand imply that higher med care prices will cause people to cut back some, but they will not much, so the total amount spent on care will go up.
Elasticity can be used for other things besides quantities and prices. The elasticity of health status with respect to medical care expenditure is the percentage difference in health status divided by the percentage difference in medical care expenditure. The cause (e.g. expenditure change) is in the denominator. The effect (health status change) is in the numerator.
The advantage of elasticity over slope is that, as mentined, elasticities
are numbers with no units.
A disadvantage of elasticity is the other side of its advantage:
The unitless number obscures the problems with measuring the quantities
in the equation, such as health status.
What determines how high demand is (i.e. how much medical care will
be bought at various prices):
Physical condition
Social factors 
affect need for care as perceived by individual and society 
Income
Insurance 
affects how much the person can pay 
Physical and cultural/demographic factors can vary regionally or even locally.
Regarding income: The usual finding is that medical spending goes up as income goes up, but less than proportionally. In other words, the income elasticity of med care demand is between 0 and 1.