What it is
The second lesser field is lexicography in its classical sense, abhidhāna, the science of names. Its concern is not what a word means but how many words there are for one meaning, and which to choose. A poet writing in metre needs the sun to have a dozen names of different lengths and flavours, “maker of day”, “friend of the lotus”, “the thousand-rayed”, and the lexicon is where those names are kept.
Where it comes from
The source is the Amarakośa, the Immortal Treasury, a Sanskrit thesaurus in verse that was translated into Tibetan in the early fourteenth century and studied alongside the Mirror of Poetry. Long before it, the translators of the canon had compiled the Mahāvyutpatti to fix how Tibetan would render Sanskrit terms, and the vocabulary of the whole literary language descends from that work. The great modern dictionaries, above all the Great Tibetan–Chinese Dictionary of 1985, are the field’s present form.
Tibetan also keeps a second vocabulary for respect, the honorific register, zhe sa. To eat, to go, to see, the head, the hand, the house, each has an ordinary word and an honorific one, and a speaker chooses by who is spoken to and who is spoken of. It is learned by ear in childhood, and it is one of the first things to fade when a language is spoken only at home.
What is at stake
Vocabulary is what a diaspora loses first. Words for things the new country does differently are replaced by the new country’s words; the honorific register thins because there are fewer occasions to use it; the literary vocabulary, which was never spoken, is not read. A language can keep its grammar and lose half its words, and be much poorer for it.
On Genla
Lexicon holds the Immortal Treasury, honorific speech, vocabulary by theme, the history of the dictionaries, and the reverse dictionary that lets a learner go from meaning to word.