Any human or metahuman with the genetic ability to transition themselves and other objects - typically a starship - into and out of Am. While the effective area of influence of most Navigators includes a space suit and possibly a toolkit, physical contact with an alloy mesh colloquially known as a "jumpnet" will expand this area of influence to cover a starship and all materials and persons contained therein (and possibly nearby, if the object is completely inside the field of influence).
The ability - and the strength of the ability - is hereditary. If one parent has the genes, the ability is present - if both parents have the genes, the ability is present and much stronger. A Navigator with two Navigator parents can inherently function at a lower level of Am than a Navigator with a single Navigator parent. While the ability has been widely "inexpertly" bred for, an enormous amount of research has gone into the Navigator genes, and the ability can to some degree be "grafted" into those born without it. While no "Needle Navigator" has ever matched the capability of a naturally born Navigator, gene therapy has been aggressively pursued as a means of keeping the prices of the various Guilds, Unions, and Associations of Navigators in check.
While the ability is in constant demand, Navigators with pedigree can command fees, bonuses and benefits well above the standard rates.
Physically, the ability expresses itself in eye color, ranging from brown with flecks of dull orange to a bright orange with flecks of white and dark orange. It is a commonly held belief that the brighter a Navigator's eyes, the more powerful the Navigator's ability.
It is an established medical fact that those with Navigation genes have a much higher incidence of all types of brain cancer, with incidences particular to the pineal gland and vomeronasal organ running as high as 1,200% above the average. Navigators are also prone to absence seizures (aka "spacing out"), manifesting around puberty and trailing off in early adulthood.
At the time of its discovery, the Navigation ability was estimated to be present in 1 out of every 900 million people. Thanks to aggressive breeding and gene therapy, as of SY 590 the ratio is estimated to be 1 in 50 million.
Michael West - A third generation Navigator naturally bred from genetically engineered stock.
John West - Inherited the ability from Michael West and his mother, amplified in utero by the Addicaine program.
Naomi Houston - Inheritance unknown.
Greg Auriga may have some degree of Navigation ability. However, he has yet to be shown Transitioning a ship into or out of Am. The ability to Transition equipment between realities is a learned skill - the Navigation genes aren't necessary. They are, however, required to transition into Am*.
Ornix Khan may have some degree of Navigation ability. Like Greg, this is inferred from his ability to transfer a vehicle between realities inside a gravity well.
* Am isn't a reality in the traditional sense, and the Veil isn't Am, in a literal sense.