Strong authentication is a notion with several unofficial definitions; is not standardized in the security literature.
Often, strong authentication is associated with two-factor authentication or more generally multi-factor authentication. Soliciting multiple answers to challenge questions may be considered strong authentication but, unless the process also retrieves 'something you have' or 'something you are', it would not be considered multi-factor. The FFIEC issued supplemental guidance on this subject in August 2006, in which they clarified, "By definition true multifactor authentication requires the use of solutions from two or more of the three categories of factors. Using multiple solutions from the same category ... would not constitute multifactor authentication."
Another commonly found class of definitions relates to a cryptographic process, or more precisely authentication based on a challenge response protocol. This type of definition is found in the Handbook of applied cryptography.[1] This type of definition does not necessarily relate to two-factor authentication, since the secret key used in a challenge-response authentication scheme can be simply derived from a password (one factor).
A third class of definitions says that strong authentication is any form of authentication in which the verification is accomplished without the transmission of a password. This is the case for example with the definition found in the Fermilab documentation.[2]
Thus, the term strong authentication can be used as long as the notion strong is defined in the context of use.