I12n

implementation (begins with i, ends with n, 12 letters in between)

Read Also:

I 131 uptake test
I 131 uptake test n. A test of thyroid function in which iodine-131 is given orally and 24 hours later the amount present in the thyroid gland is measured and compared with normal levels.
I18n
internationalisation internationalization (begins with i, ends with n, 18 letters in between)
I2o
Intelligent Input/Output