Navy Strength is a term that’s used to describe the alcoholic strength of a gin or rum. It’s not protected by law or a regulated term, but for Gin it is widely accepted as 57% ABV, while in Rum, it is 54.5% ABV.
The reason for this difference is convoluted, but to paraphrase… Bartholomew Sikes invented an accurate hydrometer in 1816 and declared 100 proof at just over 57% alcohol by volume. Sikes' new scale was adopted in law in 1818 but the British Navy conducted its own tests to establish the strength their rum should be issued at, and declared that 54.5% ABV was the number they were going to adopt as standard “Navy Strength”.
As there are many records of Rum being sold at Navy Strength, it’s clear that this the official ABV of the Navy and a part of the Rum category’s heritage.
Gin as a category is different due to the fact that there are little to no records to show the ABV it was sold at (just that Gin was indeed sold to the Navy by the hundreds of gallons). The only remaining brand that had a connection to that era and who still produced their gin, Plymouth Gin (who incidentally coined the phrase Navy Strength for the category in a marketing decision to place it on label in the late 90’s), is bottled at 57% ABV. Combine this % with being the logical ABV based on the Sikes hydrometer and this became the adopted norm for a new generation of craft gin distillers.