Lewis & Short

2. sŏlārium, ii (sc. vectigal), n. [solum], a ground-rent, Dig. 43, 8, 2, § 17; 7, 1, 7, § 2; 13, 7, 17; 30, 1, 39, § 5; Inscr. Orell. 39; cf. Becker, Antiq. 3, 2, p. 121, n. 614.