Correct term for percentage in decimal form

Solution 1:

I am partial to what Omnomnomnom mentioned in a comment: it is a proportion, and the authoritative Oxford English Dictionary supports this terminology (snippet produced below):

Solution 2:

5 years later...
While proportion is a good choice, it is not correct. Percentage in its decimal form is a proportion but a proportion is not necessarily a decimal formated percentage. It can be 0.25, 25%, 25/100 or 1/4 or whatever else that describes a proportion. Same goes with other similar words, such as index or ratio. Unfortunately, I could not find a single word that explicitly means "percentage in its decimal form", so one has to choose one of those and move on, or use a descriptive/compound name.

Solution 3:

I don't know that there is a single word to express what you want to say. It's a "percentage as a decimal" or as Thanasis Mattis said "percentage in its decimal form". You could go with "Decimal Percentage" but that's not officially defined as an actual word. You could also use "Decimal Fraction" which represents a fraction as a decimal but not necessarily a percentage (See definition below).

As mentioned by Thanasis Mattas a percentage is a portion but a portion isn't a percentage. Thus, I wouldn't use "proportion".

"Decimal Fraction" as defined by Meriam Webster : a fraction (such as .25 = ²⁵/₁₀₀ or .025 = ²⁵/₁₀₀₀) or mixed number (such as 3.025 = 3²⁵/₁₀₀₀) in which the denominator is a power of 10 usually expressed by use of the decimal point