Jamaica is an island nation of the Greater Antilles, 234 km (146 mi) in length and as much as 80 km (50 mi) in width situated in the Caribbean Sea. It is about 620 km (385 mi) northeast of the Central American mainland, 145 km (90 mi) south of Cuba, and 190 km (120 mi) west of the island of Hispaniola, on which Haiti and the Dominican Republic are situated.
Formerly a Spanish possession known as Santiago, it was later colonised by the English, and became the British Empire's Crown colony of Jamaica. Following it independence in 1962, it became a Commonwealth realm and remains so to this day.
Jamaica's status in this novel is open to debate, as the frontispiece map does not label the island. It does seem to be within Holy Alliance borders, so Spain must have reacquired it sometime after the POD.
It is not entirely clear who rules Jamaica after 1944. The map in Second Contact suggests that Jamaica is an independent country, as its colour does not match any of the empires listed in the map key, yet in the text only three independent countries in the Caribbean are listed: Cuba, the Dominican Republic, and Haiti. It may be that Jamaica retains a nominal connection to the British Empire while exercising true independence.