21 Aquilae is a solitary^[14] variable star in the equatorial constellation of Aquila. It has the variable star designation V1288 Aql; 21 Aquilae is its Flamsteed designation. This object is visible to the naked eye as a dim, blue-white hued star with a baseline apparent visual magnitude of about 5.1. The star is located at a distance of around 680 light-years (210 parsecs) from Earth, give or take a 20 light-year margin of error. It is moving closer to the Earth with a heliocentric radial velocity of –5 km/s.^[6]

The stellar classification of this star is B8 II-III, with the luminosity class of II-III suggesting that the spectrum displays elements of both a giant star and a bright giant. It is a chemically peculiar star of the Mercury-Manganese type (CP3),^[10] although some catalogues consider that status to be doubtful.^[15] This is a probable Alpha² Canum Venaticorum variable that ranges in visual magnitude from 5.06 down to 5.16.^[3] The star is radiating 704 times the luminosity of the Sun from its photosphere at an effective temperature of 12,014 K; this gives it the blue-white glow of a B-type star.^[16]

21 Aquilae is catalogued as an optical double star, having a 12th magnitude companion 37″ away as of 2010. It was first identified as a double star by John Herschel.^[17] The companion is a distant background object.^[18]