A single tattoo needle is a small-gauge (26-22ga), solid needle with an extremely sharp point. It is often formed by the tattoo artist himself by filing steel wire, although commercially made tattoo needles are available. This needle is then attached to a needle bar, which is itself attached to the reciprocating mechanism in the tattoo machine. The modern tattoo machine can be attributed to Samuel O'Reilly.
While single needles are sometimes used for extremely fine line work, it is far more common for multiple needles to be combined into groups to improve their performance. This is usually accomplished by soldering or welding them to a small block of steel. The entire group is then sterilized using an autoclave. More commonly, it is this group of needles that is referred to as a "tattoo needle."
There are many configurations of needle groups, ranging from small groups of three or four needles in a round configuration (used for fine work) up to broad, linear arrangements of 8 to 12 needles or more (used for shading, including large "Magnum" configurations).
Note: While the above is true for "modern" Western tattooing, it is not necessarily true for traditional tattooing in other cultures. Single-needle hand tattooing is traditional throughout the Polynesian Islands.