present participle of add on

(medicine) Initialism of Adverse Drug Effect. / (medicine) Initialism of antibody-dependent enhancement.

plural of AD

(tennis) advantage to the server

(physics, mathematics, cosmology) Initialism of anti de Sitter.

As much as desired, to one's fill, without restriction.

Characterized by doing something ad libitum.

At pleasure. / At will. / To whatever extent. / Extemporaneously.

Extemporaneous, impromptu.

To perform without script. / To perform without preparation. / To perform extemporaneously.