limen
Concise Oxford English Dictionary © 2008 Oxford University Press:
limen /ˈlʌɪmɛn, ˈliː-/
▶noun (pl. limens or limina /ˈlʌɪmɪnə, ˈliː-/) Psychology a threshold below which a stimulus is not perceived or distinguished from another.
– origin C17: from L., ‘threshold’.
'limen' also found in these Oxford entries:

