1e-10
is a floating-point literal with the value 1 to the power minus 10.
The Go Programming Language Specification
A floating-point literal is a decimal representation of a floating-point constant. It has an integer part, a decimal point, a fractional part, and an exponent part. The integer and fractional part comprise decimal digits; the exponent part is an e or E followed by an optionally signed decimal exponent. One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.
float_lit = decimals "." [ decimals ] [ exponent ] | decimals exponent | "." decimals [ exponent ] . decimals = decimal_digit { decimal_digit } . exponent = ( "e" | "E" ) [ "+" | "-" ] decimals . 0. 72.40 072.40 // == 72.40 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5