The term you are looking for is "Voltage Regulator".
A voltage regulator takes a higher input voltage and scales it down to a lower voltage.
There are two main types of voltage regulator:
These act like an automatic variable resistor keeping the output voltage constant regardless of current draw. They are very simple to use, very cheap, but also inefficient.
(Also known as "Buck" regulators) These use PWM to chop the DC voltage into a waveform that represents a percentage of the incoming voltage. The output is then filtered and smoothed. They are much harder to work with than a linear regulator, but they are much much more efficient.
Ideally for a battery source you want to work with a switching regulator as your battery will last longer. You can buy pre-built adjustable switching regulators quite cheap off eBay.