Amateur / ham — a licensed operator in the amateur radio service.
ATV — amateur television.
Balun / unun — transformer between balanced and unbalanced (or unbalanced and unbalanced) feeds.
Beam — a directional antenna, usually a Yagi.
Break — spoken to interrupt an ongoing conversation, usually because you have something urgent.
Call sign — the unique identifier issued by the FCC; yours, and required identification.
CQ — a general call to any station.
CW — continuous wave; Morse code.
DX — distant station, usually meaning another country.
Elmer — a mentor. Being one is the most valued role in the hobby.
FT8 — a weak-signal digital mode; see FT8.
Grid square — Maidenhead locator, e.g. EM75, identifying your location for VHF and satellite work.
Hamfest — a swap meet, forum programme and social event combined.
HT — handheld transceiver.
LoTW — Logbook of the World, ARRL's contact confirmation system.
Net — a scheduled on-air meeting; see nets.
QRP — low power operation, 5 W or less.
QSL — confirmation of a contact, or the card that records it.
QSO — a contact between two stations.
Rag chew — a long, casual conversation.
Repeater — a station that retransmits signals to extend range.
Rig — a transceiver.
RST — the readability/strength/tone signal report.
Shack — wherever your station lives, however grand or humble.
Silent key (SK) — an operator who has died.
Simplex — direct radio-to-radio operation without a repeater.
SWR — standing wave ratio; see feedline and SWR.
VE / VEC — volunteer examiner and volunteer examiner coordinator.
73 — best regards. Never "73s".
88 — love and kisses; used sparingly and traditionally.