Where can I find a canonical definition of equanimity (upekkha) in the Theravada buddhist texts?
From SN 36.31:
"Now, O monks, what is worldly equanimity? There are these five cords of sensual desire: forms cognizable by the eye... tangibles cognizable by the body that are wished for and desired, agreeable and endearing, associated with sense desire and alluring. It is the equanimity that arises with regard to these five cords of sense desire which is called 'worldly equanimity.'
"Now, what is unworldy equanimity? With the abandoning of pleasure and pain, and with the previous disappearance of gladness and sadness, a monk enters upon and abides in the fourth meditative absorption, which has neither pain-nor-pleasure and has purity of mindfulness due to equanimity. This is called 'unworldly equanimity.'
"And what is the still greater unworldly equanimity? When a taint-free monk looks upon his mind that is freed of greed, freed of hatred and freed of delusion, then there arises equanimity. This is called a 'still greater unworldly equanimity."
There's a chapter about the Brahmaviharas in the Visuddhimagga -- chapter IX, which starts on page 291 of this PDF: Path of Purification (Visuddhimagga). The section on Equanimity starts on page 310.
The Visuddhimagga is a Theravada text, I'm not sure whether it's "canonical" (Wikipedia says, "It is considered the most important Theravada text outside of the Tipitaka canon of scriptures").