1.  admittance(n = noun.attribute) access, accession, admission, entree - the right to enter;
is a kind of right
has particulars: door
Derived form verb admit₃

2.  admittance(n = noun.act) admission - the act of admitting someone to enter; "the surgery was performed on his second admission to the clinic"
is a kind of entering, entrance, entry, incoming, ingress
has particulars: readmission, matric, matriculation

1.  The act of admitting. [1913 Webster]

2.  Permission to enter; the power or right of entrance; also, actual entrance; reception. [1913 Webster]
"To gain admittance into the house." [1913 Webster]
"He desires admittance to the king." [1913 Webster]
"To give admittance to a thought of fear." [1913 Webster]

3.  Concession; admission; allowance; as, the admittance of an argument. Sir T. Browne. [1913 Webster]

4.  Admissibility. Shak. [1913 Webster]

5.  The act of giving possession of a copyhold estate. Bouvier. [1913 Webster]

   The reciprocal of impedance. [Webster 1913 Suppl.]

admittance, n.
1 the right or process of admitting or being admitted, usu. to a place (no admittance except on business).
2 Electr. the reciprocal of impedance.

Usage:
A more formal and technical word than admission.

N  reception, admission, admittance, entree, importation, introduction, intromission, immission, ingestion, imbibation, introception, absorption, ingurgitation, inhalation, suction, sucking, eating, drinking, insertion, interjection, introit, admitting, admitted, admissable, absorbent.