A "Zombie ETF" is a term used to describe an exchange-traded fund (ETF) that has seen a decline in assets under management (AUM) and trading volume, but remains open and continues to trade. These ETFs are often referred to as "zombie" because they persist despite having limited resources and support from investors.Zombie ETFs can result from a number of factors, including poor performance, changes in investor preferences, competition from newer and more popular ETFs, and a lack of marketing or promotion by the issuer.Despite the decline in assets and trading volume, zombie ETFs can continue to trade on stock exchanges and may still be held by investors who bought the ETF in the past. However, the continuation of these ETFs can lead to potential problems for investors, such as reduced liquidity, higher bid-ask spreads, and increased tracking error due to lower trading volume.In some cases, ETF issuers may choose to close down zombie ETFs and return the remaining assets to investors. Alternatively, the ETF issuer may choose to merge the zombie ETF with another ETF in order to reduce costs and improve liquidity.In conclusion, a zombie ETF is an ETF that has seen a decline in assets and trading volume but remains open and continues to trade. While these ETFs can persist, they can also lead to potential problems for investors and may be closed or merged by the issuer.