An altarpiece with two wings or side panels is called a

A reliquary is a container used to hold and display sacred relics, often associated with saints or holy figures. While it may be ornately designed, it does not refer to a specific type of altarpiece with wings. Instead, it serves a different purpose within religious practices, focusing on the preservation and veneration of relics rather than the visual storytelling associated with triptychs.

A fresco cycle refers to a series of fresco paintings that are typically done on walls or ceilings, depicting a continuous narrative or theme. While frescoes can enhance altars, they do not involve the panel structure of an altarpiece with wings and thus do not fit the description provided in the question.

A predella is a platform or base that supports an altarpiece, often containing smaller narrative scenes or figures beneath the main panels. While it is related to altarpieces, it does not describe the structure of having two wings or side panels, which is the focus of the question.

A mural is a large painting or artwork applied directly to a wall or ceiling. While it can serve a decorative or thematic purpose in a religious setting, it does not incorporate the panel format that characterizes a triptych, making it an incorrect choice in this context.