Abstruse, adj.  difficult to understand, hidden.  It should not be misused for abstract.

Accident, n.

1)      Metaphysics.  Something whose essence requires naturally that it exist in another being; a being or a being; a mere modification or attribute of another being; being in a qualified sense; being inhering in another being as in a subject of existence; one of the nine modes in which substance is determined in its being; ens entis.  In the plural, accidents are often referred to as appearances, phenomena, or species.

Antonym—substance.

  1. Absolute Accident,
    1. one that is really distinct from the subject in which is inheres, as opposed to a modal accident.
    2. one that immediately affects the substance to which is belongs as opposed to a modal accident that immediately affects another accident; thus, quantity as distinguished from shape.
  2. Intrinsic accident, one that really modifies a subject, and so is not merely an extrinsic change or name; an accidental form.
  3. Metaphysical accident, any accident in the nine categories of accidental being.
  4. Modal accident,
    1. a state affecting the substance, but which is thought of by some scholastic philosophers as not really distinct from the substance or other accidents in the being; e.g., sitting, standing. 
    2. one that immediately inheres in or affects other accidents. 
    3. a mode of being.  See MODE.
  5. Physical Accident, an absolute accident.

2)      Causal sense.  What is unforeseen or unintended; the result of chance. 

3)      Ethics.  A circumstance of a human act.  See CIRCUMSTANCE.

4)      Logic.  An attribute belonging to some nature but not constituting its essence or a part of its essence.  It is sometimes called a logical accident.

  1. Contingent (logical) accident, an attribute that is not characteristic of or essential to a nature, but may be present or absent in different members of the same species; e.g., white color in human skin.
  2. Proper accident, a characteristic or distinctive accident essentially belonging to or necessarily resulting from some essence, and so found in all members of the species; a propery or proprium.  See PREDICABLE

Ref.–Topics, I, c. 5, ST, I-II, 7, a. 1; 17, a. 9 ad 2; 53, a. 2 ad 3; 110, a. 2 ad 3; III, 77, a. 1 ad 2.

Accidental, adj.

  1. Non-Essential; not necessary; not always or usually connected with another.
  2. merely associated with or concomitant.
  3. unforeseen; unintended; beside the intention of the agent or even contrary to intention; marked by chance.
  4. per accidens, q.v.
  5. pertaining to the accidents, not to the substance.

Uses–accidental cause, change, difference, event, form, result, sensible, unit.