Decimal Point
The decimal point is the dot in a decimal number that separates the whole number part from the fractional part.
Definition
The decimal point is the small dot in a number like $3.14$ or $0.5$: everything to the left of the dot is a whole number, and everything to the right is less than one whole. Formally, it separates the integer part of a number from its fractional part in base-ten notation, with each position to the right representing a successively smaller power of $10$ ($10^{-1}$, $10^{-2}$, $10^{-3}$, etc.), and in scientific notation the decimal point is always placed after the first nonzero digit. More generally, the decimal point marks the boundary between non-negative and negative exponents of the base in any positional numeral system; the value of the digit $d$ at position $n$ (counting from the point, positive to the left) is $d \cdot b^n$ in base $b$, and other bases use analogous "radix points."
Example
In $12.75$, the decimal point separates $12$ (twelve whole things) from $.75$ (seventy-five hundredths, or $3/4$), the dividing line between "wholes" and "parts." A number like $2{,}450$ has an implied decimal point after the last zero ($2450. = 2450.0$), and moving it left step by step gives $245.0$, $24.50$, $2.450$, $0.2450$, matching scientific notation $2.45 \times 10^3$. In base $2$, $1011.01 = 1 \cdot 2^3 + 0 \cdot 2^2 + 1 \cdot 2^1 + 1 \cdot 2^0 + 0 \cdot 2^{-1} + 1 \cdot 2^{-2} = 8+2+1+0.25 = 11.25$ in base $10$, the binary "radix point" playing the exact role of the decimal point.
Key Insight
Moving the decimal point one place to the right multiplies by $10$; moving it one place left divides by $10$, so the decimal point is not just a dot, it is the key to the entire positional value system. Different countries use different decimal separators, the United States uses a period while many European countries use a comma, a seemingly minor notational choice with real practical consequences for software and international standards. Floating-point representation in computers is essentially scientific notation in base $2$: the IEEE 754 standard specifies a radix point and a fixed number of binary digits, which is why computers cannot represent $0.1$ exactly in binary, causing the rounding errors familiar to every programmer.