It has been pointed out that in its common usage the term 'dysgenics' lacks biological validity.
For example, the decline of average intelligence in a population is often described as a dysgenic change, on the basis that a decline in intelligence is (it can be argued) a bad thing on the whole for human society.
However, it could be argued that insofar as a decline in intelligence was due to a differential selection against higher intelligence people (for example, by a chosen reduction in fertility) and in favour of lower intelligence people (who perhaps cannot or will not use contraception) - then this is just 'natural selection as usual' - and indeed the fitness of the population is being enhanced under conditions where fertility rates are either very low, sometimes at sub-replacement/ long-term extinction levels.
Insofar as this is what is happening with the decline of intelligence then it is indeed just natural -selection-as-usual in that genetic mutations leading to adaptations are being handed-on from parents to their offspring.
Better adapted parents - i.e. those parents resistant to the fertility-suppressing effects of modernity - are producing a higher proportion of offspring (in the next generation) than are parents who are susceptible to fertility suppression.
Fitter parents (fitter in terms of the actual environment) have fitter children.
But insofar as the decline in intelligence is due to an accumulation of deleterious mutations (and the vast majority of new mutations will be more-or-less deleterious), then this is real dysgenics: objectively measurable, and biologically distinct from natural-selection-as-usual.
This is because with mutation accumulation, the parent is not transmitting adaptations to the offspring, but newly-occurred genetic damage. The parent does not share the mutational damage which is suffered by the offspring - since that genetic damage has occurred during the process of reproduction.
Since the new genetic mutations have not been inherited from a parent, and parents are not handing-on adaptations - then mutation accumulation is not a consequence of natural selection.
So in principle mutation accumulation is a biologically-objective dysgenic process (while differential selection of heritable traits is not) - and mutation accumulation could be measured phenotypically in terms of damage to adaptations, or genetically in terms of mutational damage to those suites of genes which underlie phenotypic adaptations.
For example, objectively and biologically dysgenic change from mutation accumulation, would include new (not inherited) mutations which led to infertility, or blindness, or fatal genetic diseases of childhood - since these phenotypic changes are objective; or else (if known) detectable as the genetic changes which underlie adaptations such as sexual attraction mate selection, fertility, child care, vision, or the functionality of any major essential organ system such as blood, cardiovascular or respiratory.
With mutation accumulation children are less fit than parents.
Or, offspring are less fit than parents with reference to the parental environment.
(It is necessary to add this rider, because in modern conditions the less fit offspring have experienced a different, and more favourable, environment than their parents. So, the functional impairment of several generations of offspring has been concealed by a 'softer' and more supportive environment, as society became richer per capita, and mortality rates fell. )
In conclusion, dysgenics can be used in a non-biological and subjective way - to refer to any disapproved-of genetic change in a population; or in a rigorous, objective and biologically-valid way - to describe a non-hereditary generational reduction in fitness in a population: the incremental loss of functional adaptations in a population.
Reference: The essence of this idea came from, and should be attributed to, Michael A Woodley.