Least Significant Bit. Least Significant Byte. (Depends on use.)

1

I assume you mean a binary representation of a number.
The "least significant bit" (usually the one to the far right but in some languages it has another placement) is "ones"
the next most significant bit are the twos
The third most significant bit are the fours
etc.

So if your number is 37
there is one 32 (the sixth most significant bit)
no 16's (the fifth most significant bit)
no 8's (the fourth most significant bit)
one 4 (the third most significant bit)
no 2's (the second most significant bit)
one 1 (the least most significant bit)

if we are to fill an 8 bit "word " we get:
0010 0101

Least significant bit.