Is 'indirect effect' the same as 'mediation'? Is it correct to use 'indirect effect' and 'mediation' interchangeably in all situations? I mean if we know that A influences B and B influences C. Can we conclude that the effect is mediated by B without performing a mediation test?  
 A: No, "mediation" and "indirect effect" are not synonymous. For example, when analyzing complex causal systems in which any variable in the system at time $t$ contributes causally to every variable in the system at times $>t$, the term "mediator" is largely meaningless, whereas indirect effects may or may not exist due both to the qualitative structure of the system, and due to the quantitative magnitude of direct causal effects withing that structure (see, for example, Levins, R. (1974). The qualitative analysis of partially specified systems. Annals of the New York Academy of Sciences, 231:123–138.)
gung's answer assumes a terminal causal model, where the analysis attempts to explain or predict a final causal outcome, typically in a limited set of variables. By contrast complex causal models attempt to explain system behaviors.
A: There seem to be two distinct aspects to this question:

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*Are the terms 'indirect effect' and 'mediation' synonymous?
I would say largely yes (edit: assuming--cf. @Alexis' answer--that you are referring to a terminal causal model).  Bear in mind that you can have full or partial mediation.  That is, the effect of A on C can flow only through B, or partly through B but also partly directly.  It is less clear to me how one would use 'indirect effect' in the latter case.  To my ear it sounds more like full mediation.


*If we know a-priori that A causes B, and B in turn causes C, do we need a mediation test?
No.  If you know that this is the case a-priori, then testing is silly and redundant.  This has to do with the nature of statistical testing; there is nothing special about mediation here:  We can also say that there is no need to do a $t$-test (for example), to determine if the mean of a variable differs by grouping after having done a median split.
A: The word mediation is inappropriate when the influence from A to C is not significant. However, you can say that A influences C indirectly through B, i.e. indirect effect.
