"Hierarchical linear modeling (HLM) is a particular regression model that is designed to take into account the hierarchical or nested structure of the data. HLM is also known as multi-level modeling, linear mixed-effects model, or covariance components model."

Source: Goldstein, A. and Leyland, A. (2001). Multilevel Modelling of health statistics. Series: Wiley series in probability and statistics. 

Cited after: Matsuyama, Y. (2013). Hierarchical Linear Modeling (HLM). Gellman, M.D., Turner, J.R. (eds) Encyclopedia of Behavioral Medicine. 
Online: https://link.springer.com/referenceworkentry/10.1007/978-1-4419-1005-9_407#:~:text=Definition,Leyland%20%26%20Goldstein%2C%202001).