Systematic difference of participants in the study from the population from which they were selected due to selective loss of participants.
Systematic deviation of a measure of the effect of intervention from true value due to different attrition rate of subjects in the trial. Different loss of subjects in intervention and comparison group (e.g. withdrawals, dropouts, protocol deviators) may change the characteristics of these groups and outcomes irrespective of the studied intervention.
Jüni P, Egger M. Empirical evidence of attrition bias in clinical trials. Int J Epidemiol. 2005 Feb;34(1):87-8.