[Virology] Is TCID50 a dilution? an amount of virus?

[mlcepedah from neo.tamu.edu]

Dear All,

I'm trying to write a detailed protocol for TCID50 determinations.  Some
of the consulted references define TCID50 as the required dilution to
achieve 50% infection of the tested units, and therefore the value is
given in terms of 10 elevated to a negative fraction (i.e. 10^-3.5/ml). 
However, other papers use TCID50 as the number of virus particles in a
sample required to infect 50% of the tested units (i.e. 10^3.5/ml). These
are values calculated either by the Karber method or the Reed-Muench
method, and everybody seems to use different approaches.
I'll appreciate very much if someone can help me define TCID50.

Thanks,

Martha