You can ask both Matlab and Octave about the meaning of any predefined
function by using the `help` command. So to ask about `gammai`
in Octave type:

octave:38> help gammai gammai is the function defined from: /afs/ovpit.indiana.edu/sun4x_56/gnu/share/octave/2.0.13/m/specfun/gammai.m usage: gammai (a, x) Compute the incomplete gamma function gammai (a, x) = (\int_0^x exp(-t) t^(a-1) dt) / gamma(a). If a is scalar, then gammai (a, x) is returned for each element of x and vice versa. If neither a nor x is scalar, the sizes of a and x must agree, and gammainc is applied for corresponding elements of x and a. Additional help for builtin functions, operators, and variables is available in the on-line version of the manual. Use the command `help -i <topic>' to search the manual index. Help and information about Octave is also available on the WWW at http://www.che.wisc.edu/octave/octave.html and via the help-octave@bevo.che.wisc.edu mailing list. octave:39>

>> help gammainc GAMMAINC Incomplete gamma function. Y = GAMMAINC(X,A) evaluates the incomplete gamma function for corresponding elements of X and A. X and A must be real and the same size (or either can be a scalar). A must also be non-negative. The incomplete gamma function is defined as: gammainc(x,a) = 1 ./ gamma(a) .* integral from 0 to x of t^(a-1) exp(-t) dt For any a>=0, as x approaches infinity, gammainc(x,a) approaches 1. For small x and a, gammainc(x,a) ~= x^a, so gammainc(0,0) = 1. See also GAMMA, GAMMALN. >>

For compatibility reasons, compatibility with Matlab that is,
Octave also has function `gammainc`:

octave:39> help gammainc gammainc is a builtin function gammainc (x, a) Compute the incomplete gamma function gammainc(x,a) = (\int_0^x exp(-t) t^(a-1) dt) / gamma(a). If a is scalar, then gammainc(x,a) is returned for each element of x and vice versa. If neither a nor x is scalar, the sizes of a and x must agree, and gammainc is applied for corresponding elements of x and a. Additional help for builtin functions, operators, and variables is available in the on-line version of the manual. Use the command `help -i <topic>' to search the manual index. Help and information about Octave is also available on the WWW at http://www.che.wisc.edu/octave/octave.html and via the help-octave@bevo.che.wisc.edu mailing list. octave:40>