The imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil.

"What is a Datum Line ?"

That is the chord of any airfoil.

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What is the line of incidence

It is the Cord line, and with reference to the centerline of the fuselage, the angle measurement makes the angle of incidence.

You are right in that it is the cord but not in that the angle it makes with the fuse is angle of incidence. The angle of incidence is the angle between the chord of the stab and the cord of the wing and can theoretically be at any angle to the fuselage center line.

OK, it is CHORD.

INCIDENCE is the angle of the wing with respect to the reference line through the fuselage...the datum line from front to rear. The horizontal stabilizer can also have an angle of incidence.

DECALAGE is the commonly-used term for the angular relationship between the wing and horizontal stabilizer. This has also been referred to as LONGITUDINAL DIHEDRAL.

ANGLE OF ATTACK is the angle between the wing and the oncoming airstream when in flight. "Flight" starts when the aircraft begins its takeoff run, and ends when it completes the landing roll...at least as far as this discussion is concerned.

Word.

`Decalage' actually originally meant the difference in rigged incidence of the planes (wings) of a biplane...monoplanes couldn't have any decalage...
Evan, WB #12.

Decalage in French = Shift in English

http://en.wiktionary.org/wiki/decalage

Décalage vers le rouge = Shift towards the red

There you go...And then there's 'cabane', 'nother French word (they designed the aeroplane, you know) which was a pyramidal structure built above the wing of a monoplane to which that wing was braced...in other words, biplanes (which have struts, both wing and centre section) don't have cabanes, don't need them...but I notice that most American biplanes seem to have them...
Evan, WB #12.