Zero period ground accelaration

13:22 Jun 15, 2016 

I found a few definitions for "zero period acceleration" closely related to this term. The first definition is much more formal compared to the others.

Zero period acceleration corresponds, on a response spectrum, to the frequencies which cause the structure to trace ground accelerations without amplification of motion.

Zero period acceleration or ZPA is the response acceleration value when the structure is very rigid and there is no amplification of ground spectra because of structure. ( i.e response acceleration when time period of structure is zero). In any response spectra the horizontal part of graph is at ZPA. ( This is usually taken as response acceleration value at frequency of 33Hz).

Zero period acceleration Implies max acceleration experienced by a
structure having zero natural period (T=0).

I also can't understand these terms used in the first definition.
Frequency: The first definition uses the term "frequency" to define "zero period acceleration". For me "frequency" means something like this: the total number of a number such as "8" in a set of numbers like 8,3,4,2,8

I also cannot understand the meaning of "trace" in this definition.