Style drift occurs if an investment manager moves away from the investment mix that's appropriate to a mutual fund or managed account portfolio based on the portfolio's objectives and style.
Such a drift typically occurs if the core portfolio is providing disappointing returns while other investments in the marketplace are performing better. In this case, a manager may feel pressure to increase the bottom line. The drift may also occur inadvertently if some of a portfolio's underlying investments take on different characteristics. For example, a small company may become a mid-sized company or a value investment may increase substantially in price.
The danger of style drift from a portfolio perspective is that you might end up owning investments that are more or less risky than you intended or that exposed you unexpectedly to portfolio overlap.
Advocates of index investing cite style drift as a key disadvantage of actively managed funds, pointing out that index investments stay true to their style no matter what's happening in the economy.