Alternate, opposite, or whorled are all terms for various types of leaf arrangements on a stem, which is also known as phyllotaxy.

Plants with alternate leaves have a single leaf at each node, like this Netleaf Hackberry (Celtis laevigata var. reticulata).

Plants with opposite leaves have a pair of leaves at each node, like this Confederate Jasmine (Trachelospermum jasminoides) shown below.

Plants with whorled leaves have 3 or more leaves around the stem at each node, like this Common Bedstraw or Stickywilly (Galium aparine).