Assured member   - a member of the CAcert community who has 50+ APs (COD13 Assurance Policy #2.3)
  (AP: Prospective Member with created account + Member)
Unassured member - a member of the CAcert community who has <50 APs (COD13 Assurance Policy #2.3)
  (AP: Assured Member + Prospective Assurer + Assurer)
Root CA certificate - the main certificate of a CA (Main Root)
Subordinate CA certificate (Subroot, intermediate CA certificate)
Issuer DN - Distinguished Name of the issuer authority
Subject DN - Distinguished Name of the subject, for which the certificate is issued
...
and 1.3 from CPS COD6
Certification Authority
Registration Authority
Subscriber
Relying Party
Member
Arbitrator
Vendor
Non-Related Person
...
abbrev.
CA   (Certification Authority)
OCSP (On-line Certificate Status Protocol) - the protocol and server giving the quick informaton about revoked certificates based on their serial numbers
CSR  (Certificate Signing Request) - user's utilities, as OpenSSL, XCA, and others, generate pairs of keys; one key of each pair is claimed as the private key and saved on user's machine; the other key with some added information about the user is usually given the PEM (Base64 coded) format and serves as the requeat to issue a certificate
WoT  (Web of Trust) - 
CRL  (Certificate Revoke List) - the list of revoked certificates
AP   (Assurance Points) - points a member can collect by means of assurance appointments
RSA: Rivest–Shamir–Adleman cryptography, developed 1978
ECC: Elliptic Curve Cryptography - gives greater cryptographic strength then RSA at equivalent key size, developed 1985
DN   Distinguished Name
X.509v3