https://doi.org/10.1351/goldbook.P04859
This term is used in different, sometimes contradictory ways; four are listed below.
  1. The geometric property of an
    achiral
    object (or spatial arrangement of points or atoms) which is capable of becoming
    chiral
    in a single
    desymmetrization
    step. An
    achiral
    molecular entity, or a part of it considered on its own, is thus called prochiral if it can be made
    chiral
    by the replacement of an existing atom (or
    achiral
    group) by a different one. An
    achiral
    object which is capable of becoming
    chiral
    in two
    desymmetrization
    steps is sometimes described as proprochiral. For example the proprochiral CH3CO2H becomes prochiral as CH2DCO2H and
    chiral
    as CHDTCO2H.
  2. The term prochirality also applies to an
    achiral
    molecule or entity which contains a trigonal system and which can be made
    chiral
    by the addition to the trigonal system of a new atom or
    achiral
    group. For example addition of hydrogen to one of the
    enantiotopic
    faces of the prochiral ketone CH3CH2COCH3 gives one of the enantiomers of the
    chiral
    alcohol CH3CH2CHOHCH3; the addition of CN to one of the
    diastereotopic
    faces of the
    chiral
    aldehyde shown below converts it into one of the
    diastereoisomers
    of the cyanohydrin. The two faces of the trigonal system may be described as Re and Si.
  3. The term prochiral also applies to a tetrahedral atom of an
    achiral
    or
    chiral
    molecule which is bonded to two
    stereoheterotopic
    groups. For example, the prochiral molecule CH3CH2OH can be converted into the
    chiral
    molecule CH3CHDOH by the isotopic replacement of one of the two
    enantiotopic
    hydrogen atoms of the
    methylene
    group. The carbon atom of the
    methylene
    group is called prochiral. The prochiral molecule HO2CCH2CHOHCH2CO2H can be converted into a
    chiral
    product by esterification of one of the two
    enantiotopic
    –CH2CO2H groups. The carbon atom of the CHOH group is called prochiral. The
    chiral
    molecule CH3CHOHCH2CH3 can be converted into one of the
    diastereoisomers
    of CH3CHOHCHDCH3 by the isotopic replacement of one of the two
    diastereotopic
    hydrogen atoms of the
    methylene
    group. The carbon atom of the
    methylene
    group is called prochiral. The
    stereoheterotopic
    groups in these cases may be described as pro-R or pro-S. Reference to the two
    stereoheterotopic
    groups themselves as prochiral, although common, is strongly discouraged.
    See:
    chirality centre
  4. The term prochirality is also applied to the
    enantiotopic
    faces of a trigonal system.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2213 [Terms] [Paper]