Passive voice is a grammatical construction in which the recipient of an action becomes the grammatical subject of the sentence, so the focus shifts from who did something to what was done. The classic contrast is "The ball was kicked by John" (passive) versus "John kicked the ball" (active). Structurally, English passive voice is formed with a form of the verb to be plus the past participle — was kicked, is written, have been reviewed — and optionally a "by …" phrase naming the actor, which can be omitted entirely.
That ability to drop the actor is exactly why passive voice is sometimes chosen deliberately. Use it when the doer is unknown ("The store was burgled overnight"), irrelevant ("The samples were refrigerated for 24 hours"), or best left unstated. Scientific and technical writing leans on the passive to keep the focus on process and results rather than on the researcher, and bureaucratic writing uses it to avoid assigning blame ("Mistakes were made").
The common criticism is that passive constructions are usually longer, more abstract, and less direct than their active equivalents, and heavy passive use makes prose feel evasive and harder to read — which is why most editing guides recommend defaulting to active voice. Two nuances worth knowing: passive voice is a legitimate grammatical choice, not an error, so the advice is "use it purposefully", not "never use it". And automated grammar checkers over-flag it — a "was" followed by a participle is not always passive, and any style tool's passive count should be read as a prompt to review, not an automatic fault. Tightening passive sentences into active ones typically lowers word and syllable counts, nudging readability scores in a favourable direction; check the effect in our Word Counter.