subvisible

adjective

sub·​vis·​i·​ble ˌsəb-ˈvi-zə-bəl 

variants or sub-visible

: not visible without the aid of special instruments

subvisible particles

subvisible cirrus clouds

For Great Lakes fishermen, the immediate threat is not the thumb-size grown-ups but the subvisible larvae they spawn by the tens of thousands. Off the shores of Toledo and Cleveland and near Leamington, Ont., a single cubic meter of water often contains a half-million mussel larvae.

—Geoffrey Cowley

Word History

First Known Use

1862, in the meaning defined above