The term implantation temperature dip is often used to refer to a luteal phase dip that occurs around the time of expected implantation (7-10 days past ovulation).
An implantation dip is a short-term decrease in the temperature that happen around the time of expected implantation (about 6-12 after ovulation). Seeing an implantation dip on your fertility temperature curve does not always mean that you are pregnant nor does an absence of a dip mean that you are not pregnant.
The expression "implantation dip" is often used because of the timing of the dip and because it seems that this pattern appears with greater frequency on pregnancy charts than non-pregnancy charts. There may be some reasons why this dip is more often seen in pregnancy chart:
The estrogen hormone lowers temperatures. Estrogen levels have been found to be higher when there is a pregnancy so this may also explain the greater frequency with which we see the implantation dip on pregnancy charts. However, an estrogen surge does also happen about a week after ovulation normally when there is no pregnancy and this could cause a temperature dip at this time without a pregnancy.