origin: (13th Century) Greek; kanōn= rule", Latin; canonicus= one living under a rule
Dr. Who as personal jesus, stories falling outside the accepted episodic happenstance are to be properly labeled "alternate universe" or AU. "Headcanon" as concepts existing only in one's brain or desire. "Retcon" short for retroactive continuity to call back canon and reestablish it under a new telling or in light of new information.
"Canon" is a word you hear used a lot lately, though mostly in terms of comic books or television storylines, the original usage of the word is biblical however. So, the definition expands beyond the accepted or degreed works & laws considered sacred, or the individual clergyman belonging to a church, to be an established set of rules or principles in works and practices of all kinds (by which something is judged); a criterion; that which is considered authentic.
In addition, there is a form of music referred to as "canon", where two or more independent melodic lines (or "voices) are injected into a piece, overlapping, until they morph into one consistent sound - such as the thoroughly famous: Pachelbel Canon in D Major.
origin: (before 900) Welsh; ywen, Irish eó= stem or shaft
On Palm Sunday, this past Sunday and the Sunday before Easter, Christians honor the day Jesus rode into Jerusalem on a Donkey (the animal symbolizing peace, unlike the horse ridden into battle). The people are said to have laid down cloaks and branches of trees upon his path. As everyone in the world does not have access to palms or palm trees, several other substitutions are used: box, yew, willow, and olive.
I grew up with yew trees actually and never realized it until now, a community surrounded by trees with rigid green needles that never lost their color -- evergreens -- that grew little red berries that us young children would pretend to gobble (as parents had forbidden eating them as poisonous) or collect for decorative purposes in our little games and the established canon of our childhood rituals. Long ago, the wood of a yew tree, both strong and pliable, was seen ideal for making bows in archery too.
The fact that this year's Palm Sunday is followed immediately by a "blood moon" (or total lunar eclipse) feels especially auspicious, even if the appreciation is purely academic or scientific. Don't worry if you miss it, there are three more chances of seeing the moon turn red, or "a grouping of four" known throughout science and mathematics as the Greek-described tetrad; a bonus word for my tardiness!