The northern pike (Esox lucius), also known as jack, northern or simply pike, is a species of carnivorous fish of the genus Esox (pikes). It is commonly found in fresh waters of the Northern Hemisphere (i.e. it has a holarctic distribution).

Pike can grow to a relatively large size. Their average length is about 40–55 cm (16–22 in), with maximum recorded lengths of up to 150 cm (59 in) and maximum weights of 28.4 kg (63 lb). The IGFA currently recognises a 25 kg (55 lb) pike caught by Lothar Louis on Greffern Lake, Germany, on 16 October 1986, as the all-tackle world-record holding northern pike. Northern pike grow to larger sizes in Eurasia than in North America, and in coastal Eurasian regions than inland ones.

The northern pike gets its common name from its resemblance to the pole-weapon known as the pike (from the Middle English for 'pointed'). Names such as pike, pickerel, Lakes pike, great northern pike, jackfish, jack, slough shark snake, slimer, slough snake, gator, and hammer handle are used around the globe. Other names incorporating things such as "pointy nose" and "long head" are common as well. The name "Northern" is very popular in the Midwest (in the U.S. Upper Midwest and in the Canadian provinces of Alberta and Manitoba. Numerous other names are or have been in use. Its earlier common name, the luci (now lucy) or luce when fully grown, was used to form its taxonomic name (Esox lucius) and is used in heraldry.

Northern pike are most often olive green, shading from yellow to white along the belly. The flank is marked with short, light bar-like spots and a few to many dark spots on the fins. Sometimes, the fins are reddish. Younger pike have yellow stripes along a green body; later, the stripes divide into light spots and the body turns from green to olive green. The lower half of the gill cover lacks scales, and it has large sensory pores on its head and on the underside of its lower jaw which are part of the lateral line system. Unlike the similar-looking and closely related muskellunge, the northern pike has light markings on a dark body background and fewer than six sensory pores on the underside of each side of the lower jaw.

A hybrid between northern pike and muskellunge is known as a tiger muskellunge (Esox masquinongy × lucius or Esox lucius × masquinongy, depending on the sex of each of the contributing species). In the hybrids, the males are invariably sterile, while females are often fertile, and may back-cross with the parent species. Another form of northern pike, the silver pike, is not a subspecies but rather a mutation that occurs in scattered populations. Silver pike, sometimes called silver muskellunge, lack the rows of spots and appear silver, white, or silvery-blue in color. When ill, silver pike have been known to display a somewhat purplish hue; long illness is also the most common cause of male sterility.

In Italy, the newly identified species Esox cisalpinus ("southern pike") was long thought to be a color variation of the northern pike, but was in 2011 announced to be a species of its own.

Northern pike in North America seldom reach the size of their European counterparts; one of the largest specimens known was a 21 kg (46 lb) specimen from New York. It was caught in Great Sacandaga Lake on 15 September 1940 by Peter Dubuc. Reports of far larger pike have been made, but these are either misidentifications of the pike's larger relative, the muskellunge, or simply have not been properly documented and belong in the realm of legend. Northern Pike are capable of reaching a length of 140 centimeters and a weight of 25 kilograms. The largest Northern Pike on video was caught in the Netherlands, measuring 138 cm in length and over 23 kg in weight.

As northern pike grow longer, they increase in weight, and the relationship between length and weight is not linear. The relationship between total length (L, in inches) and total weight (W, in pounds) for nearly all species of fish can be expressed by an equation of the form