Alkyd medium is an alternative to using oil in oil painting. Alkyd medium is made from a synthetic resign and dries faster than oil mediums but slow than acrylics, whilst retaining many of the benefits of oil mediums.
Alkyd medium is popular as it removes the need for a solvent when oil painting.
I personally do not bother with alkyd mediums, as I prefer the feel and finish of oil mediums and do not feel the slow drying times of oil medium is an issue as I like to work alla prima (wet on wet).
However, if you enjoy the feel of acrylic paints but are put off by the disadvantages such as the very fast drying times, the plastic like finish and the darkening color as it dries, then you may want to try alkyd mediums.