chemically peculiar star

A chemically peculiar star (CP star, often what is meant by peculiar star) is a star with an unusual SED, indicating the presence of metals not expected in such stars, particularly if it does not fit the sequence of spectral features used to identify spectral classes. The common types are thought to stem from conditions at star formation, but other types may result from interactions of binary stars. Four common patterns:

• Am-Fm aka CP1, e.g., Am star.
• Bp-Ap aka CP2, e.g., Ap star.
• HgMn aka CP3 - mercury-manganese star, prominent mercury absorption line.
• He-weak aka CP4

Other types:

• Lambda Boötis star (λ Boo).
• He-rich.
• Be star.
• barium star - unexpected barium absorption lines.
• Alpha² Canum Venaticorum variable (ACV).
• lead star (overabundance of lead and bismuth).
• CH star - strong methylidyne absorption bands.

There are some post-main-sequence stars that would seem to fall in this category (S-type star, carbon star, PG 1159 star), but I believe the "peculiar" label is most often used regarding main sequence stars.