All the terms given are related to the process of reproduction in a human being.
According to national human genome research institute, the definition of these terms are given below:
A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm. Ova and sperm are haploid cells, with each cell carrying only one copy of each chromosome.
An allele is one of two or more versions of DNA sequence (a single base or a segment of bases) at a given genomic location. An individual inherits two alleles, one from each parent, for any given genomic location where such variation exists. If the two alleles are the same, the individual is homozygous for that allele.
The word ‘diploid” is formed from the words ‘di’, meaning “two”, and ‘ploidy’ which refers to the number of sets of chromosomes in a cell ; the term is defined as that which possesses two sets of each chromosome.
A diploid cell has two complete sets of chromosomes. Most cells in humans are diploid, comprising 23 chromosome pairs, so 46 chromosomes in total. This is 22 pairs of autosomes and a pair of sex chromosomes. One copy of each chromosome pair came from the individual's mother and the other from the individual's father.
On the other hand, haploid refers to the presence of a single set of chromosomes in an organism's cells. Sexually reproducing organisms are however diploid (having two sets of chromosomes, one from each parent). In humans, only the egg and sperm cells are haploid i.e., the gametes.
According to the Genomics Education Programme of England, homologous chromosomes are two chromosomes in a pair – normally one inherited from the mother and one from the father. For example, the two copies of Chromosome 1 in a cell would be referred to as homologous chromosomes. The chromosomal pair contains the maternal as well as the paternal chromatid of the same length and gene position, and are joined by the centromere.
( No internal links available for further reference. Go to the websites of the institutes mentioned in the answer).