[pl.] in the word book means the abbreviation of the plural form.

plural

English [?pl?r?l] American [?pl?r?l]? adj. plural n. plural; plural form; plural word. Its abbreviation is pl. (abbr. pl.).

Example:

Words?using?the?singular?or?plural?number?also?include?the?plural?or?singular?number. Use singular or plural words Also includes the plural or singular.

Extended information:

Rules for changing nouns into the plural:

If a noun refers to one or the same thing, it takes the singular form, such as pen, dog, tree , fact, church, kiss. If it means two or more such things, you need to use the plural form of the noun, such as pens, dogs, trees, facts, churches, kisses. To form a plural noun, most of them are added after the singular form. -s or -es, there are the following 6 situations:

1. In general, add s

2. When ending with s, sh, ch, x, add es, read /iz/ .

3. It ends with a consonant letter + y, change y to i and add es, and pronounce /iz/.

4. If the word ends with a consonant + o, if it is alive, add es; if it is not alive, add s.

5. Ending with f or fe, remove f and fe becomes ves.

Reference material: Plural nouns - Baidu Encyclopedia