Definition of 'Plum'
A choice investment, or one that has outperformed other comparable investments. While there are no specific quantitative criteria to define a plum, it should have outperformed over a reasonable period of time without being perceived as being unduly risky.
Investopedia explains 'Plum'
An asset class may be a plum investment in one period and a lemon (the antithesis of a plum) in another. For example, large-cap technology stocks were plum investments in the 1990s, but turned out to be lemons in the first decade of the new Millennium, when US Treasury bonds were among the plums.
In general, stocks that have posted consistent returns for many years would qualify as plum investments. A well diversified portfolio of blue-chips would generally contain a number of plums, but may also contain a few lemons - stocks that are chronic underperformers with poor or negative rates of return.