Dictionary Definition :
Definition[1]
:
以一龜之腹眼巧值浮木孔之難,喻指受身為人、值佛在世、遇聞佛法等稀有難逢之際遇。謂大海有一龜,其腹部具一眼,隨波浮游,遇大木之穴則乘之,偶因風吹來,遂得覆此木,龜仰向,其腹之一眼當於浮木之孔而見日月之光。同理,眾生欲值佛世或得人身,亦如此龜腹部之一眼值遇浮木孔而見日光之難得。法華經莊嚴王品(大九‧六○上):「佛難得值,如優曇鉢羅華;又如一眼之龜,值浮木孔(中略)。諸佛難值,時亦難遇。」(參閱「盲龜浮木」3457)
Source
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佛光電子大辭典
Page
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p66
Definition[2]
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A sea turtle with only one eye, and that underneath, entered a hollow in a floating log; the log, tossed by the waves, happened to roll over, whereupon the turtle momentarily saw the sun and moon; an illustration of the rareness of the appearance of a Buddha; also of the difficulty of being reborn as a man.
Source
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A Dictionary of Chinese Buddhist Terms, William Edward Soothill and Lewis Hodous
Definition[3]
:
(譬喻)大海有龜,腹有一眼。隨波浮遊於中,遇大木之穴則乘之,偶風吹來,遂覆此木,龜仰向其腹之一眼,當於浮木之孔,見日月之光,此譬希有難逢之際遇(見十住論之鼇眼持頭),盲龜之喻少異之。法華經莊嚴王品曰:「佛難得值,如優波羅華,又如一眼之龜值浮木孔。」十住論八曰:「人身難得,如大海中有一眼鱉頭入板孔。」
Source
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丁福保《佛學大辭典》