Taiko-Tenno is an appellation of emperor, which is applied during a period from the demise of the Emperor to the grant of Tsuigo (posthumous title).
Taiko means 'great behavior,' where the word is used to pay honor to the demised Emperor.
Also, it is said to have a meaning of 'the Emperor who passed away and never returns'.
According to a current system (which is known as 'Isse ichigen no sei' [literally, the practice of assigning one era name to one emperor]), Gengo (an era name) during the Emperor's reign usually becomes that Emperor's posthumous title. However, it should be noted that a posthumous title is conferred during the Tsuigo hogo no gi (a ceremony for dedicating that title) which precedes the Taiso no rei (funeral service of a Japanese emperor).
Therefore, the appellation ''Taiko-Tenno'' is used to refer to the demised emperor on official gazettes before the dedication of his posthumous title in the ceremony.
In fact, a sentence like 'A Buddhist memorial service for the Taiko-Tenno is held' appears in journals published by several religious organizations.
For instance, in the case where a Buddhist organization provides memorial service for the demised emperor before his posthumous title is conferred, they often use a temporary ihai (Buddhist mortuary tablet) made of plain wood or paper wherever necessary.