1. (verb) To speak, perform, or act without preparation or written script; to improvise
The actor had to ad lib when he forgot his lines.
O ator teve que improvisar quando esqueceu suas falas.
2. (noun) Something said or performed without preparation; an improvisation
His funny ad lib got a big laugh from the audience.
Seu improviso engraçado arrancou risadas da plateia.
3. (adjective) Done or made without preparation; improvised
The comedian's ad lib jokes were hilarious.
As piadas improvisadas do comediante foram hilariantes.