When referencing stock markets and publicly traded equities, "Epsilon" can refer to several market listings depending on the exchange. The most prominent direct ticker is Epsilon Energy Ltd. (NASDAQ: EPSN), an independent energy exploration and natural gas production company operating onshore in North America. Alternatively, in quantitative financial modeling and portfolio analysis, Greek letters like epsilon are used to denote error terms, unsystematic residual risk, or idiosyncratic returns that are unexplained by standard capital asset pricing models and macroeconomic market betas.