What is "intrinsic equation of a curve"

Intrinsic \In*trin"sic\ ([i^]n*tr[i^]n"s[i^]k), a. [L. intrinsecus inward, on the inside; intra within + secus otherwise, beside; akin to E. second: cf. F. intrins[`e]que. See Inter-, Second, and cf. Extrinsic.]

1. Inward; internal; hence, true; genuine; real; essential; inherent; not merely apparent or accidental; -- opposed to extrinsic; as, the intrinsic value of gold or silver; the intrinsic merit of an action; the intrinsic worth or goodness of a person.

   He was better qualified than they to estimate justly the intrinsic value of Grecian philosophy and refinement.
   --I. Taylor.

2. (Anat.) Included wholly within an organ or limb, as certain groups of muscles; -- opposed to extrinsic.

   Intrinsic energy of a body (Physics), the work it can do in virtue of its actual condition, without any supply of energy from without.

   Intrinsic equation of a curve (Geom.), the equation which expresses the relation which the length of a curve, measured from a given point of it, to a movable point, has to the angle which the tangent to the curve at the movable point makes with a fixed line.

   Intrinsic value. See the Note under Value, n.

   Syn: Inherent; innate; natural; real; genuine.