1a land mass surrounded by water on three sides
2 a transitional area between land and water systems that supports aquatic plants
3 an area where freshwater and salt water meet
4 the principal land mass of a continent or country
5 a frozen layer of soil beneath the earth's surface
6 a small, low island of coral or sand
a cay
b peninsula
c mainland
d wetland
e permafrost
f estuary

Explanation:

1 a land mass surrounded by water on three sides

Peninsula

A peninsula is a piece of land usually in a water body that is not completely surrounded by water and it is connected to land by an isthmus

2 a transitional area between land and water systems that supports aquatic plants

Wetland

Wetlands are transitional environments where water always floods the land seasonally or sometimes throughout the year. This supports the growth of diverse aquatic plant. They are also vital in cultivation of certain food crops like rice.

3 an area where freshwater and salt water meet

Estuary

An estuary is a transitional environment where freshwater meets with salt water. The water here is brackish water.

4 the principal land mass of a continent or country

Mainland

The land mass in a continent or country is the mainland. It contains the continental shield and craton.

5 a frozen layer of soil beneath the earth's surface

Permafrost

Permafrost is common in polar and temperate regions. Here, the soil remains frozen throughout the year.

6 a small, low island of coral or sand

cay

 A cay is a small coral or sand island.