Physical presence refers to the time that you have actually been physically present in the USA. If you are a permanent resident 18 years of age or older, then you must have been physically present in the United States for at least 30 months during the last five years (or 18 months during the last three years, if married to a U.S. citizen) before you can apply for naturalization.
Question: What is the difference between physical presence and continuous residence?
Answer: Physical presence refers to the total number of days you were inside the United States and does not include the time you spent outside the USA. Each day you spend outside the U.S. takes away from your physical presence total.
If you were away from the U.S. for a long period of time or if you take many short trips outside the United States, then you may not meet the physical presence requirement. To calculate your physical presence, you should count the number of days you have been a permanent resident for the last 5 years and subtract the number of days you have traveled outside of the USA. This includes short trips to Canada and Mexico. For example, if you go to Mexico for a weekend, you must include the trip when counting how many days you spent out of the country.
Continuous residence is the total time you have resided as a permanent resident in the United States before applying for naturalization. If you have spent too much time outside the United States during a single trip, you may break your continuous residence.