A doxology is a short hymn praising God. While it's usually added to the end of other hymns, Ben Weatherstaff suggests that Colin use a classic Protestant doxology as a standalone hymn as Colin develops his system of Magic.
Doxology Term Timeline in The Secret Garden
The timeline below shows where the term Doxology appears in The Secret Garden. The colored dots and icons indicate which themes are associated with that appearance.
...to recite something joyful and thankful. Ben Weatherstaff grunts that Colin might consider singing the Doxology. Colin has never heard of it before as he's never been to church, but Dickon... (full context)