In finance, a shell corporation is a corporation that is "just a shell," that is a corporation without any business activities or assets. In other words, it is a corporation in a legal sense, but in no other sense.
Shell corporations are used for a variety of perfectly legal purposes, such as launching a startup, executing a triangular merger, allowing a previously private corporation to become publicly listed on a stock exchange, initiating a hostile takeover, and many others.
However, shell corporation generally has a skeezy nuance, because in some cases criminal actors use shell corporations for a variety of nefarious purposes, such as covering up illegal activity, misleading regulators, obfuscating true company ownership, money laundering, and tax evasion, among others.