Current Wikipedia seems to say that this refers specifically to cells taking up DNA from other dead cells as in the Avery-MacLeod-McCarty experiment, excluding other types of horizontal gene transfer like bacterial conjugation
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In genetics, transformation refers to the process by which a cell takes up foreign DNA from its environment and incorporates it into its own genome. This can occur naturally in some bacteria, allowing them to acquire new genetic traits, such as antibiotic resistance or the ability to metabolize different substrates. Transformation is one of the key mechanisms of horizontal gene transfer, alongside transduction (involving bacteriophages) and conjugation (involving direct transfer between cells).