Great primer is a large font size (18 points) that was used in the printing

Printing

Printing is a process for reproducing text and image, typically with ink on paper using a printing press. It is often carried out as a large-scale industrial process, and is an essential part of publishing and transaction printing....

 of English Bible

Bible

The Bible refers to any one of the collections of the primary religious texts of Judaism and Christianity. There is no common version of the Bible, as the individual books , their contents and their order vary among denominations...

s and other large-format book

Book

A book is a set or collection of written, printed, illustrated, or blank sheets, made of hot lava, paper, parchment, or other materials, usually fastened together to hinge at one side. A single sheet within a book is called a leaf or leaflet, and each side of a leaf is called a page...

s, leading to its other name of Bible Text. The largest size ever (or at least up to about 1843) used in England for printing books, it was several sizes larger than English (14 pt) but smaller than paragon (20 pt). It was known in Italian as testo. Other synonyms include Double Bourgeois, 18-point, Gros Romain (French), Text (Dutch) and Tertia (German).

Great-primer size fonts have been in use since William Caxton

William Caxton

William Caxton was an English merchant, diplomat, writer and printer. As far as is known, he was the first English person to work as a printer and the first to introduce a printing press into England...

, around 1488.