'''Functional analogs''' (or '''functional analogues''') are entities (models, representations, etc.) that can be replaced, to fulfill the same function. When the entities in question are formally represented by [[black box]]es, the concept of ''analog'' is related to "same behavior": they take the same output sequence when submitted to the same input sequence.
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== Systems engineering==
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[[Image:Thevenin equivalent.png|frame|right|Any ''black box'' containing resistances only, with voltage and current sources, can be replaced to a [[Thévenin's theorem|Thévenin equivalent circuit]], to show the same behavior.]]
[[Analogical models]] are used in a method of representing a ‘[[Open system (systems theory)|target system]]’ by another, more understandable or analysable system. Two systems have "analog functions" (see illustration) if the black box representation of both can be the same.
