Strictly, the point in phage-bacterium interaction at which the phage genome becomes located within the bacterium's cytoplasm and continuing until phage and/or bacterium are no longer functionally intact.
More generally, i.e., as applied to non-phage viruses or other pathogens, see infection.
Phage infections in most instances require irreversible attachment of the phage virion to the surface of a bacterium (adsorption) but strictly occurs only upon successful movement (translocation) of phage nucleic acid into the cytoplasm of the same bacterium, thereby giving rise to either phage production, phage reduction, or phage destruction (productive, reductive, and destructive infections, respectively).
Though infections strictly can be viewed as phage adsorptions that pass at least through the phage-genome translocation stage, less strictly the word is often used at least partially synonymously with adsorption or attachment.
This is the definition from Adams (1959), p. 440, "Used loosely to indicate attachment of phage to bacterium with at least some effects on the cell. One speaks, perhaps improperly, of infection by ghosts of T2 because the cell may be killed, though injection and phage growth are impossible."
This is from Delbrück (1945), p. 131: "We use the word 'infection' of a bacterium by a virus to designate the fact that a bacterium has adsorbed a virus particle. We do not imply that the infecting particle necessarily grows. Thus, we will speak of 'multiple infection' when several virus particles of the same strain become adsorbed on one bacterium, and of 'mixed infection' when virus particles of different strains become adsorbed on the same bacterium. In all these cases very probably only one of the infecting particles grows. This use of the word 'infection,' though differing from medical usage, is etymologically correct and has been used previously in bacterial virus work."
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