An apparently defective prophage that can serve as an antibacterial agent, where head and DNA is absent from an otherwise adsorption-competent phage particle.
The term tailocin has been proposed as an alternative to high-molecular weight pyocin or bacteriocin. The basically consist of tailed phage tails but without the rest of the phage.
From Gill and Young (2011), p. 377: "Many bacteria have taken advantage of the cellular leakage associated with the phage DNA ejection process by evolving 'tailocins', a term we propose here for what were originally called 'high-molecular weight pyocins' when they were first identified in Pseudomonas strains... Basically, these are the apparent products of degenerate prophage elements which lack head genes; when the tailocin determinants are expressed, headless phage particles are produced that appear to kill the target cells by the same hole-forming process used for phage DNA ejection, except that no DNA ever goes through to cause re-sealing of the hole. The new term tailocin is justified because the same entities have now been found in many other bacterial species, and the continued use of the pyocin term would be misleading."
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