Markowitz's definition of an efficient portfolio is one that minimizes portfolio risk for a given level of expected return. He therefore calls portfolios along the efficient frontier "frontier portfolios" as well as "efficient portfolios".
There also exist well-performing strategies called heuristic portfolios including the equally-weighted (1/N) portfolio, and inverse-volatility portfolio (IVP). I know the 1/N portfolio doesn't lie on the efficient frontier, but is known to often outperform all frontier (efficient) portfolios out-of-sample. There's also no comment about the IVP being efficient. Can we call these two heuristic portfolios efficient portfolios?
optional question: Are the following strategies also efficient portfolios or no?
- hierarchical risk parity portfolio (HRP)
- maximum diversification portfolio (MDP)
- maximum decorrelation portfolio