Once upon a time I reviewed Richard Mabey's lovely Nature Cure. Ever since, I have periodically tried to remember some of the excellent new words it taught me. The only one I thought I had succeeded in nailing down was 'ideolect', meaning the distinctive language and verbal tics a sub-societal group develops and sometimes uses in order to enhance its own cohesiveness. As it transpires, the word is 'ecolect'.*
The one I am always maddened to forget is 'isophene'. This is a line (like an isobar, etc.) linking the dates of first flowerings of a particular plant.
Other good ones included tarpans and koniks, which are types of wild horses.
*Coined by Hugh Sykes Davies